def BIBD_5q_5_for_q_prime_power(q):
r"""
Return a `(5q,5,1)`-BIBD with `q\equiv 1\pmod 4` a prime power.
See Theorem 24 [ClaytonSmith]_.
INPUT:
- ``q`` (integer) -- a prime power such that `q\equiv 1\pmod 4`.
EXAMPLES::
sage: from sage.combinat.designs.bibd import BIBD_5q_5_for_q_prime_power
sage: for q in [25, 45, 65, 85, 125, 145, 185, 205, 305, 405, 605]: # long time
....: _ = BIBD_5q_5_for_q_prime_power(q/5) # long time
"""
from sage.rings.finite_rings.constructor import FiniteField
if q%4 != 1 or not is_prime_power(q):
raise ValueError("q is not a prime power or q%4!=1.")
d = (q-1)/4
B = []
F = FiniteField(q,'x')
a = F.primitive_element()
L = {b:i for i,b in enumerate(F)}
for b in L:
B.append([i*q + L[b] for i in range(5)])
for i in range(5):
for j in range(d):
B.append([ i*q + L[b ],
((i+1)%5)*q + L[ a**j+b ],
((i+1)%5)*q + L[-a**j+b ],
((i+4)%5)*q + L[ a**(j+d)+b],
((i+4)%5)*q + L[-a**(j+d)+b],
])
return B
def skew_hadamard_matrix(n,existence=False, skew_normalize=True, check=True):
r"""
Tries to construct a skew Hadamard matrix
A Hadamard matrix `H` is called skew if `H=S-I`, for `I` the identity matrix
and `-S=S^\top`. Currently constructions from Section 14.1 of [Ha83]_ and few
more exotic ones are implemented.
INPUT:
- ``n`` (integer) -- dimension of the matrix
- ``existence`` (boolean) -- whether to build the matrix or merely query if
a construction is available in Sage. When set to ``True``, the function
returns:
- ``True`` -- meaning that Sage knows how to build the matrix
- ``Unknown`` -- meaning that Sage does not know how to build the
matrix, but that the design may exist (see :mod:`sage.misc.unknown`).
- ``False`` -- meaning that the matrix does not exist.
- ``skew_normalize`` (boolean) -- whether to make the 1st row all-one, and
adjust the 1st column accordingly. Set to ``True`` by default.
- ``check`` (boolean) -- whether to check that output is correct before
returning it. As this is expected to be useless (but we are cautious
guys), you may want to disable it whenever you want speed. Set to ``True``
by default.
EXAMPLES::
sage: from sage.combinat.matrices.hadamard_matrix import skew_hadamard_matrix
sage: skew_hadamard_matrix(12).det()
2985984
sage: 12^6
2985984
sage: skew_hadamard_matrix(1)
[1]
sage: skew_hadamard_matrix(2)
[ 1 1]
[-1 1]
TESTS::
sage: skew_hadamard_matrix(10,existence=True)
False
sage: skew_hadamard_matrix(12,existence=True)
True
sage: skew_hadamard_matrix(784,existence=True)
True
sage: skew_hadamard_matrix(10)
Traceback (most recent call last):
...
ValueError: A skew Hadamard matrix of order 10 does not exist
sage: skew_hadamard_matrix(36)
36 x 36 dense matrix over Integer Ring...
sage: skew_hadamard_matrix(36)==skew_hadamard_matrix(36,skew_normalize=False)
False
sage: skew_hadamard_matrix(52)
52 x 52 dense matrix over Integer Ring...
sage: skew_hadamard_matrix(92)
92 x 92 dense matrix over Integer Ring...
sage: skew_hadamard_matrix(816) # long time
816 x 816 dense matrix over Integer Ring...
sage: skew_hadamard_matrix(100)
Traceback (most recent call last):
...
ValueError: A skew Hadamard matrix of order 100 is not yet implemented.
sage: skew_hadamard_matrix(100,existence=True)
Unknown
REFERENCES:
.. [Ha83] \M. Hall,
Combinatorial Theory,
2nd edition,
Wiley, 1983
"""
def true():
_skew_had_cache[n]=True
return True
M = None
if existence and n in _skew_had_cache:
return True
if not(n % 4 == 0) and (n > 2):
if existence:
return False
raise ValueError("A skew Hadamard matrix of order %s does not exist" % n)
if n == 2:
if existence:
return true()
M = matrix([[1, 1], [-1, 1]])
elif n == 1:
if existence:
return true()
M = matrix([1])
elif is_prime_power(n - 1) and ((n - 1) % 4 == 3):
if existence:
return true()
M = hadamard_matrix_paleyI(n, normalize=False)
elif n % 8 == 0:
if skew_hadamard_matrix(n//2,existence=True): # (Lemma 14.1.6 in [Ha83]_)
if existence:
return true()
H = skew_hadamard_matrix(n//2,check=False)
M = block_matrix([[H,H], [-H.T,H.T]])
else: # try Williamson construction (Lemma 14.1.5 in [Ha83]_)
for d in divisors(n)[2:-2]: # skip 1, 2, n/2, and n
n1 = n//d
if is_prime_power(d - 1) and (d % 4 == 0) and (n1 % 4 == 0)\
and skew_hadamard_matrix(n1,existence=True):
if existence:
return true()
H = skew_hadamard_matrix(n1, check=False)-I(n1)
U = matrix(ZZ, d, lambda i, j: -1 if i==j==0 else\
1 if i==j==1 or (i>1 and j-1==d-i)\
else 0)
A = block_matrix([[matrix([0]), matrix(ZZ,1,d-1,[1]*(d-1))],
[ matrix(ZZ,d-1,1,[-1]*(d-1)),
_helper_payley_matrix(d-1,zero_position=0)]])+I(d)
M = A.tensor_product(I(n1))+(U*A).tensor_product(H)
break
if M is None: # try Williamson-Goethals-Seidel construction
if GS_skew_hadamard_smallcases(n, existence=True):
if existence:
return true()
M = GS_skew_hadamard_smallcases(n)
else:
if existence:
return Unknown
raise ValueError("A skew Hadamard matrix of order %s is not yet implemented." % n)
if skew_normalize:
dd = diagonal_matrix(M[0])
M = dd*M*dd
if check:
assert is_hadamard_matrix(M, normalized=False, skew=True)
if skew_normalize:
from sage.modules.free_module_element import vector
assert M[0]==vector([1]*n)
_skew_had_cache[n]=True
return M
def regular_symmetric_hadamard_matrix_with_constant_diagonal(n,e,existence=False):
r"""
Return a Regular Symmetric Hadamard Matrix with Constant Diagonal.
A Hadamard matrix is said to be *regular* if its rows all sum to the same
value.
For `\epsilon\in\{-1,+1\}`, we say that `M` is a `(n,\epsilon)-RSHCD` if
`M` is a regular symmetric Hadamard matrix with constant diagonal
`\delta\in\{-1,+1\}` and row sums all equal to `\delta \epsilon
\sqrt(n)`. For more information, see [HX10]_ or 10.5.1 in
[BH12]_. For the case `n=324`, see :func:`RSHCD_324` and [CP16]_.
INPUT:
- ``n`` (integer) -- side of the matrix
- ``e`` -- one of `-1` or `+1`, equal to the value of `\epsilon`
EXAMPLES::
sage: from sage.combinat.matrices.hadamard_matrix import regular_symmetric_hadamard_matrix_with_constant_diagonal
sage: regular_symmetric_hadamard_matrix_with_constant_diagonal(4,1)
[ 1 1 1 -1]
[ 1 1 -1 1]
[ 1 -1 1 1]
[-1 1 1 1]
sage: regular_symmetric_hadamard_matrix_with_constant_diagonal(4,-1)
[ 1 -1 -1 -1]
[-1 1 -1 -1]
[-1 -1 1 -1]
[-1 -1 -1 1]
Other hardcoded values::
sage: for n,e in [(36,1),(36,-1),(100,1),(100,-1),(196, 1)]:
....: print regular_symmetric_hadamard_matrix_with_constant_diagonal(n,e)
36 x 36 dense matrix over Integer Ring
36 x 36 dense matrix over Integer Ring
100 x 100 dense matrix over Integer Ring
100 x 100 dense matrix over Integer Ring
196 x 196 dense matrix over Integer Ring
sage: for n,e in [(324,1),(324,-1)]: # not tested - long time, tested in RSHCD_324
....: print regular_symmetric_hadamard_matrix_with_constant_diagonal(n,e) # not tested - long time
324 x 324 dense matrix over Integer Ring
324 x 324 dense matrix over Integer Ring
From two close prime powers::
sage: print regular_symmetric_hadamard_matrix_with_constant_diagonal(64,-1)
64 x 64 dense matrix over Integer Ring
Recursive construction::
sage: print regular_symmetric_hadamard_matrix_with_constant_diagonal(144,-1)
144 x 144 dense matrix over Integer Ring
REFERENCE:
.. [BH12] \A. Brouwer and W. Haemers,
Spectra of graphs,
Springer, 2012,
http://homepages.cwi.nl/~aeb/math/ipm/ipm.pdf
.. [HX10] \W. Haemers and Q. Xiang,
Strongly regular graphs with parameters `(4m^4,2m^4+m^2,m^4+m^2,m^4+m^2)` exist for all `m>1`,
European Journal of Combinatorics,
Volume 31, Issue 6, August 2010, Pages 1553-1559,
http://dx.doi.org/10.1016/j.ejc.2009.07.009.
"""
if existence and (n,e) in _rshcd_cache:
return _rshcd_cache[n,e]
from sage.graphs.strongly_regular_db import strongly_regular_graph
def true():
_rshcd_cache[n,e] = True
return True
M = None
if abs(e) != 1:
raise ValueError
if n<0:
if existence:
return False
raise ValueError
elif n == 4:
if existence:
return true()
if e == 1:
M = J(4)-2*matrix(4,[[int(i+j == 3) for i in range(4)] for j in range(4)])
else:
M = -J(4)+2*I(4)
elif n == 36:
if existence:
return true()
if e == 1:
M = strongly_regular_graph(36, 15, 6, 6).adjacency_matrix()
M = J(36) - 2*M
else:
M = strongly_regular_graph(36,14,4,6).adjacency_matrix()
M = -J(36) + 2*M + 2*I(36)
elif n == 100:
if existence:
return true()
if e == -1:
M = strongly_regular_graph(100,44,18,20).adjacency_matrix()
M = 2*M - J(100) + 2*I(100)
else:
M = strongly_regular_graph(100,45,20,20).adjacency_matrix()
M = J(100) - 2*M
elif n == 196 and e == 1:
if existence:
return true()
M = strongly_regular_graph(196,91,42,42).adjacency_matrix()
M = J(196) - 2*M
elif n == 324:
if existence:
return true()
M = RSHCD_324(e)
elif ( e == 1 and
n%16 == 0 and
is_square(n) and
is_prime_power(sqrt(n)-1) and
is_prime_power(sqrt(n)+1)):
if existence:
return true()
M = -rshcd_from_close_prime_powers(int(sqrt(n)))
# Recursive construction: the kronecker product of two RSHCD is a RSHCD
else:
from itertools import product
for n1,e1 in product(divisors(n)[1:-1],[-1,1]):
e2 = e1*e
n2 = n//n1
if (regular_symmetric_hadamard_matrix_with_constant_diagonal(n1,e1,existence=True) and
regular_symmetric_hadamard_matrix_with_constant_diagonal(n2,e2,existence=True)):
if existence:
return true()
M1 = regular_symmetric_hadamard_matrix_with_constant_diagonal(n1,e1)
M2 = regular_symmetric_hadamard_matrix_with_constant_diagonal(n2,e2)
M = M1.tensor_product(M2)
break
if M is None:
from sage.misc.unknown import Unknown
_rshcd_cache[n,e] = Unknown
if existence:
return Unknown
raise ValueError("I do not know how to build a {}-RSHCD".format((n,e)))
assert M*M.transpose() == n*I(n)
assert set(map(sum,M)) == {e*sqrt(n)}
return M
def hadamard_matrix_paleyII(n):
"""
Implements the Paley type II construction.
The Paley type II case corresponds to the case `p \cong 1 \mod{4}` for a
prime `p` (see [Hora]_).
EXAMPLES::
sage: sage.combinat.matrices.hadamard_matrix.hadamard_matrix_paleyII(12).det()
2985984
sage: 12^6
2985984
We note that the method returns a normalised Hadamard matrix ::
sage: sage.combinat.matrices.hadamard_matrix.hadamard_matrix_paleyII(12)
[ 1 1| 1 1| 1 1| 1 1| 1 1| 1 1]
[ 1 -1|-1 1|-1 1|-1 1|-1 1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 -1| 1 1|-1 -1|-1 -1| 1 1]
[ 1 1|-1 -1| 1 -1|-1 1|-1 1| 1 -1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 1| 1 -1| 1 1|-1 -1|-1 -1]
[ 1 1| 1 -1|-1 -1| 1 -1|-1 1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1|-1 -1| 1 1| 1 -1| 1 1|-1 -1]
[ 1 1|-1 1| 1 -1|-1 -1| 1 -1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1|-1 -1|-1 -1| 1 1| 1 -1| 1 1]
[ 1 1|-1 1|-1 1| 1 -1|-1 -1| 1 -1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 1|-1 -1|-1 -1| 1 1| 1 -1]
[ 1 1| 1 -1|-1 1|-1 1| 1 -1|-1 -1]
TESTS::
sage: from sage.combinat.matrices.hadamard_matrix import (hadamard_matrix_paleyII, is_hadamard_matrix)
sage: test_cases = [2*(x+1) for x in range(50) if is_prime_power(x) and x%4==1]
sage: all(is_hadamard_matrix(hadamard_matrix_paleyII(n),normalized=True,verbose=True)
....: for n in test_cases)
True
"""
q = n//2 - 1
if not(n%2==0 and is_prime_power(q) and (q % 4 == 1)):
raise ValueError("The order %s is not covered by the Paley type II construction." % n)
from sage.rings.finite_rings.finite_field_constructor import FiniteField
K = FiniteField(q,'x')
K_list = list(K)
K_list.insert(0,K.zero())
H = matrix(ZZ, [[(1 if (x-y).is_square() else -1)
for x in K_list]
for y in K_list])
for i in range(q+1):
H[0,i] = 1
H[i,0] = 1
H[i,i] = 0
tr = { 0: matrix(2,2,[ 1,-1,-1,-1]),
1: matrix(2,2,[ 1, 1, 1,-1]),
-1: matrix(2,2,[-1,-1,-1, 1])}
H = block_matrix(q+1,q+1,[tr[v] for r in H for v in r])
return normalise_hadamard(H)
def balanced_incomplete_block_design(v, k, existence=False, use_LJCR=False):
r"""
Return a BIBD of parameters `v,k`.
A Balanced Incomplete Block Design of parameters `v,k` is a collection
`\mathcal C` of `k`-subsets of `V=\{0,\dots,v-1\}` such that for any two
distinct elements `x,y\in V` there is a unique element `S\in \mathcal C`
such that `x,y\in S`.
More general definitions sometimes involve a `\lambda` parameter, and we
assume here that `\lambda=1`.
For more information on BIBD, see the
:wikipedia:`corresponding Wikipedia entry <Block_design#Definition_of_a_BIBD_.28or_2-design.29>`.
INPUT:
- ``v,k`` (integers)
- ``existence`` (boolean) -- instead of building the design, return:
- ``True`` -- meaning that Sage knows how to build the design
- ``Unknown`` -- meaning that Sage does not know how to build the
design, but that the design may exist (see :mod:`sage.misc.unknown`).
- ``False`` -- meaning that the design does not exist.
- ``use_LJCR`` (boolean) -- whether to query the La Jolla Covering
Repository for the design when Sage does not know how to build it (see
:func:`~sage.combinat.designs.covering_design.best_known_covering_design_www`). This
requires internet.
.. SEEALSO::
* :func:`steiner_triple_system`
* :func:`v_4_1_BIBD`
* :func:`v_5_1_BIBD`
TODO:
* Implement other constructions from the Handbook of Combinatorial
Designs.
EXAMPLES::
sage: designs.balanced_incomplete_block_design(7, 3).blocks()
[[0, 1, 3], [0, 2, 4], [0, 5, 6], [1, 2, 6], [1, 4, 5], [2, 3, 5], [3, 4, 6]]
sage: B = designs.balanced_incomplete_block_design(66, 6, use_LJCR=True) # optional - internet
sage: B # optional - internet
Incidence structure with 66 points and 143 blocks
sage: B.blocks() # optional - internet
[[0, 1, 2, 3, 4, 65], [0, 5, 24, 25, 39, 57], [0, 6, 27, 38, 44, 55], ...
sage: designs.balanced_incomplete_block_design(66, 6, use_LJCR=True) # optional - internet
Incidence structure with 66 points and 143 blocks
sage: designs.balanced_incomplete_block_design(216, 6)
Traceback (most recent call last):
...
NotImplementedError: I don't know how to build a (216,6,1)-BIBD!
TESTS::
sage: designs.balanced_incomplete_block_design(85,5,existence=True)
True
sage: _ = designs.balanced_incomplete_block_design(85,5)
A BIBD from a Finite Projective Plane::
sage: _ = designs.balanced_incomplete_block_design(21,5)
Some trivial BIBD::
sage: designs.balanced_incomplete_block_design(10,10)
(10,10,1)-Balanced Incomplete Block Design
sage: designs.balanced_incomplete_block_design(1,10)
(1,0,1)-Balanced Incomplete Block Design
Existence of BIBD with `k=3,4,5`::
sage: [v for v in xrange(50) if designs.balanced_incomplete_block_design(v,3,existence=True)]
[1, 3, 7, 9, 13, 15, 19, 21, 25, 27, 31, 33, 37, 39, 43, 45, 49]
sage: [v for v in xrange(100) if designs.balanced_incomplete_block_design(v,4,existence=True)]
[1, 4, 13, 16, 25, 28, 37, 40, 49, 52, 61, 64, 73, 76, 85, 88, 97]
sage: [v for v in xrange(150) if designs.balanced_incomplete_block_design(v,5,existence=True)]
[1, 5, 21, 25, 41, 45, 61, 65, 81, 85, 101, 105, 121, 125, 141, 145]
For `k > 5` there are currently very few constructions::
sage: [v for v in xrange(300) if designs.balanced_incomplete_block_design(v,6,existence=True) is True]
[1, 6, 31, 66, 76, 91, 96, 106, 111, 121, 126, 136, 141, 151, 156, 171, 181, 186, 196, 201, 211, 241, 271]
sage: [v for v in xrange(300) if designs.balanced_incomplete_block_design(v,6,existence=True) is Unknown]
[51, 61, 81, 166, 216, 226, 231, 246, 256, 261, 276, 286, 291]
Here are some constructions with `k \geq 7` and `v` a prime power::
sage: designs.balanced_incomplete_block_design(169,7)
(169,7,1)-Balanced Incomplete Block Design
sage: designs.balanced_incomplete_block_design(617,8)
(617,8,1)-Balanced Incomplete Block Design
sage: designs.balanced_incomplete_block_design(433,9)
(433,9,1)-Balanced Incomplete Block Design
sage: designs.balanced_incomplete_block_design(1171,10)
(1171,10,1)-Balanced Incomplete Block Design
And we know some inexistence results::
sage: designs.balanced_incomplete_block_design(21,6,existence=True)
False
"""
lmbd = 1
# Trivial BIBD
if v == 1:
if existence:
return True
return BalancedIncompleteBlockDesign(v, [], check=False)
if k == v:
if existence:
return True
return BalancedIncompleteBlockDesign(v, [range(v)], check=False, copy=False)
# Non-existence of BIBD
if (v < k or
k < 2 or
(v-1) % (k-1) != 0 or
(v*(v-1)) % (k*(k-1)) != 0 or
# From the Handbook of combinatorial designs:
#
# With lambda>1 other exceptions are
# (15,5,2),(21,6,2),(22,7,2),(22,8,4).
(k==6 and v in [36,46]) or
(k==7 and v == 43) or
# Fisher's inequality
(v*(v-1))/(k*(k-1)) < v):
if existence:
return False
raise EmptySetError("There exists no ({},{},{})-BIBD".format(v,k,lmbd))
if k == 2:
if existence:
return True
from itertools import combinations
return BalancedIncompleteBlockDesign(v, combinations(range(v),2), check=False, copy=True)
if k == 3:
if existence:
return v%6 == 1 or v%6 == 3
return steiner_triple_system(v)
if k == 4:
if existence:
return v%12 == 1 or v%12 == 4
return BalancedIncompleteBlockDesign(v, v_4_1_BIBD(v), copy=False)
if k == 5:
if existence:
return v%20 == 1 or v%20 == 5
return BalancedIncompleteBlockDesign(v, v_5_1_BIBD(v), copy=False)
from difference_family import difference_family
from database import BIBD_constructions
if (v,k,1) in BIBD_constructions:
if existence:
return True
return BlockDesign(v,BIBD_constructions[(v,k,1)](), copy=False)
if BIBD_from_arc_in_desarguesian_projective_plane(v,k,existence=True):
if existence:
return True
B = BIBD_from_arc_in_desarguesian_projective_plane(v,k)
return BalancedIncompleteBlockDesign(v, B, copy=False)
if BIBD_from_TD(v,k,existence=True):
if existence:
return True
return BalancedIncompleteBlockDesign(v, BIBD_from_TD(v,k), copy=False)
if v == (k-1)**2+k and is_prime_power(k-1):
if existence:
return True
from block_design import projective_plane
return BalancedIncompleteBlockDesign(v, projective_plane(k-1),copy=False)
if difference_family(v,k,existence=True):
if existence:
return True
G,D = difference_family(v,k)
return BalancedIncompleteBlockDesign(v, BIBD_from_difference_family(G,D,check=False), copy=False)
if use_LJCR:
from covering_design import best_known_covering_design_www
B = best_known_covering_design_www(v,k,2)
# Is it a BIBD or just a good covering ?
expected_n_of_blocks = binomial(v,2)/binomial(k,2)
if B.low_bd() > expected_n_of_blocks:
if existence:
return False
raise EmptySetError("There exists no ({},{},{})-BIBD".format(v,k,lmbd))
B = B.incidence_structure()
if B.num_blocks() == expected_n_of_blocks:
if existence:
return True
else:
return B
if existence:
return Unknown
else:
raise NotImplementedError("I don't know how to build a ({},{},1)-BIBD!".format(v,k))
def TaylorTwographDescendantSRG(q, clique_partition=None):
r"""
constructing the descendant graph of the Taylor's two-graph for `U_3(q)`, `q` odd
This is a strongly regular graph with parameters
`(v,k,\lambda,\mu)=(q^3, (q^2+1)(q-1)/2, (q-1)^3/4-1, (q^2+1)(q-1)/4)`
obtained as a two-graph descendant of the
:func:`Taylor's two-graph <sage.combinat.designs.twographs.taylor_twograph>` `T`.
This graph admits a partition into cliques of size `q`, which are useful in
:func:`~sage.graphs.graph_generators.GraphGenerators.TaylorTwographSRG`,
a strongly regular graph on `q^3+1` vertices in the
Seidel switching class of `T`, for which we need `(q^2+1)/2` cliques.
The cliques are the `q^2` lines on `v_0` of the projective plane containing the unital
for `U_3(q)`, and intersecting the unital (i.e. the vertices of the graph and the point
we remove) in `q+1` points. This is all taken from §7E of [BvL84]_.
INPUT:
- ``q`` -- a power of an odd prime number
- ``clique_partition`` -- if ``True``, return `q^2-1` cliques of size `q`
with empty pairwise intersection. (Removing all of them leaves a clique, too),
and the point removed from the unital.
EXAMPLES::
sage: g=graphs.TaylorTwographDescendantSRG(3); g
Taylor two-graph descendant SRG: Graph on 27 vertices
sage: g.is_strongly_regular(parameters=True)
(27, 10, 1, 5)
sage: from sage.combinat.designs.twographs import taylor_twograph
sage: T = taylor_twograph(3) # long time
sage: g.is_isomorphic(T.descendant(T.ground_set()[1])) # long time
True
sage: g=graphs.TaylorTwographDescendantSRG(5) # not tested (long time)
sage: g.is_strongly_regular(parameters=True) # not tested (long time)
(125, 52, 15, 26)
TESTS::
sage: g,l,_=graphs.TaylorTwographDescendantSRG(3,clique_partition=True)
sage: all(map(lambda x: g.is_clique(x), l))
True
sage: graphs.TaylorTwographDescendantSRG(4)
Traceback (most recent call last):
...
ValueError: q must be an odd prime power
sage: graphs.TaylorTwographDescendantSRG(6)
Traceback (most recent call last):
...
ValueError: q must be an odd prime power
"""
p, k = is_prime_power(q,get_data=True)
if k==0 or p==2:
raise ValueError('q must be an odd prime power')
from sage.schemes.projective.projective_space import ProjectiveSpace
from sage.modules.free_module_element import free_module_element as vector
from sage.rings.finite_rings.integer_mod import mod
from __builtin__ import sum
Fq = FiniteField(q**2, 'a')
PG = map(tuple,ProjectiveSpace(2, Fq))
def S(x,y):
return sum(map(lambda j: x[j]*y[2-j]**q, xrange(3)))
V = filter(lambda x: S(x,x)==0, PG) # the points of the unital
v0 = V[0]
V.remove(v0)
if mod(q,4)==1:
G = Graph([V,lambda y,z: not (S(v0,y)*S(y,z)*S(z,v0)).is_square()], loops=False)
else:
G = Graph([V,lambda y,z: (S(v0,y)*S(y,z)*S(z,v0)).is_square()], loops=False)
G.name("Taylor two-graph descendant SRG")
if clique_partition:
lines = map(lambda x: filter(lambda t: t[0]+x*t[1]==0, V),
filter(lambda z: z != 0, Fq))
return (G, lines, v0)
else:
return G
def kirkman_triple_system(v, existence=False):
r"""
Return a Kirkman Triple System on `v` points.
A Kirkman Triple System `KTS(v)` is a resolvable Steiner Triple System. It
exists if and only if `v\equiv 3\pmod{6}`.
INPUT:
- `n` (integer)
- ``existence`` (boolean; ``False`` by default) -- whether to build the
`KTS(n)` or only answer whether it exists.
.. SEEALSO::
:meth:`IncidenceStructure.is_resolvable`
EXAMPLES:
A solution to Kirkmman's original problem::
sage: kts = designs.kirkman_triple_system(15)
sage: classes = kts.is_resolvable(1)[1]
sage: names = '0123456789abcde'
sage: def to_name(r_s_t):
....: r, s, t = r_s_t
....: return ' ' + names[r] + names[s] + names[t] + ' '
sage: rows = [' '.join(('Day {}'.format(i) for i in range(1,8)))]
sage: rows.extend(' '.join(map(to_name,row)) for row in zip(*classes))
sage: print('\n'.join(rows))
Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7
07e 18e 29e 3ae 4be 5ce 6de
139 24a 35b 46c 05d 167 028
26b 03c 14d 257 368 049 15a
458 569 06a 01b 12c 23d 347
acd 7bd 78c 89d 79a 8ab 9bc
TESTS::
sage: for i in range(3,300,6):
....: _ = designs.kirkman_triple_system(i)
"""
if v % 6 != 3:
if existence:
return False
raise ValueError("There is no KTS({}) as v!=3 mod(6)".format(v))
if existence:
return False
elif v == 3:
return BalancedIncompleteBlockDesign(3, [[0, 1, 2]], k=3, lambd=1)
elif v == 9:
classes = [[[0, 1, 5], [2, 6, 7], [3, 4, 8]],
[[1, 6, 8], [3, 5, 7], [0, 2, 4]],
[[1, 4, 7], [0, 3, 6], [2, 5, 8]],
[[4, 5, 6], [0, 7, 8], [1, 2, 3]]]
KTS = BalancedIncompleteBlockDesign(
v, [tr for cl in classes for tr in cl], k=3, lambd=1, copy=False)
KTS._classes = classes
return KTS
# Construction 1.1 from [Stinson91] (originally Theorem 6 from [RCW71])
#
# For all prime powers q=1 mod 6, there exists a KTS(2q+1)
elif ((v - 1) // 2) % 6 == 1 and is_prime_power((v - 1) // 2):
from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
q = (v - 1) // 2
K = GF(q, 'x')
a = K.primitive_element()
t = (q - 1) // 6
# m is the solution of a^m=(a^t+1)/2
from sage.groups.generic import discrete_log
m = discrete_log((a**t + 1) / 2, a)
assert 2 * a**m == a**t + 1
# First parallel class
first_class = [[(0, 1), (0, 2), 'inf']]
b0 = K.one()
b1 = a**t
b2 = a**m
first_class.extend([(b0 * a**i, 1), (b1 * a**i, 1), (b2 * a**i, 2)]
for i in list(range(t)) +
list(range(2 * t, 3 * t)) +
list(range(4 * t, 5 * t)))
b0 = a**(m + t)
b1 = a**(m + 3 * t)
b2 = a**(m + 5 * t)
first_class.extend([[(b0 * a**i, 2), (b1 * a**i, 2), (b2 * a**i, 2)]
for i in range(t)])
# Action of K on the points
action = lambda v, x: (v + x[0], x[1]) if len(x) == 2 else x
# relabel to integer
relabel = {(p, x): i + (x - 1) * q
for i, p in enumerate(K) for x in [1, 2]}
relabel['inf'] = 2 * q
classes = [[[relabel[action(p, x)] for x in tr] for tr in first_class]
for p in K]
KTS = BalancedIncompleteBlockDesign(
v, [tr for cl in classes for tr in cl], k=3, lambd=1, copy=False)
KTS._classes = classes
return KTS
# Construction 1.2 from [Stinson91] (originally Theorem 5 from [RCW71])
#
# For all prime powers q=1 mod 6, there exists a KTS(3q)
elif (v // 3) % 6 == 1 and is_prime_power(v // 3):
from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
q = v // 3
K = GF(q, 'x')
a = K.primitive_element()
t = (q - 1) // 6
A0 = [(0, 0), (0, 1), (0, 2)]
B = [[(a**i, j), (a**(i + 2 * t), j), (a**(i + 4 * t), j)]
for j in range(3) for i in range(t)]
A = [[(a**i, 0), (a**(i + 2 * t), 1), (a**(i + 4 * t), 2)]
for i in range(6 * t)]
# Action of K on the points
action = lambda v, x: (v + x[0], x[1])
# relabel to integer
relabel = {(p, j): i + j * q
for i, p in enumerate(K) for j in range(3)}
B0 = [A0] + B + A[t:2 * t] + A[3 * t:4 * t] + A[5 * t:6 * t]
# Classes
classes = [[[relabel[action(p, x)] for x in tr] for tr in B0]
for p in K]
for i in list(range(t)) + list(range(2 * t, 3 * t)) + list(
range(4 * t, 5 * t)):
classes.append([[relabel[action(p, x)] for x in A[i]] for p in K])
KTS = BalancedIncompleteBlockDesign(
v, [tr for cl in classes for tr in cl], k=3, lambd=1, copy=False)
KTS._classes = classes
return KTS
else:
# This is Lemma IX.6.4 from [BJL99].
#
# This construction takes a (v,{4,7})-PBD. All points are doubled (x has
# a copy x'), and an infinite point \infty is added.
#
# On all blocks of 2*4 points we "paste" a KTS(2*4+1) using the infinite
# point, in such a way that all {x,x',infty} are set of the design. We
# do the same for blocks with 2*7 points using a KTS(2*7+1).
#
# Note that the triples of points equal to {x,x',\infty} will be added
# several times.
#
# As all those subdesigns are resolvable, each class of the KTS(n) is
# obtained by considering a set {x,x',\infty} and all sets of all
# parallel classes of the subdesign which contain this set.
# We create the small KTS(n') we need, and relabel them such that
# 01(n'-1),23(n'-1),... are blocks of the design.
gdd4 = kirkman_triple_system(9)
gdd7 = kirkman_triple_system(15)
X = [B for B in gdd4 if 8 in B]
for b in X:
b.remove(8)
X = sum(X, []) + [8]
gdd4.relabel({v: i for i, v in enumerate(X)})
gdd4 = gdd4.is_resolvable(True)[1] # the relabeled classes
X = [B for B in gdd7 if 14 in B]
for b in X:
b.remove(14)
X = sum(X, []) + [14]
gdd7.relabel({v: i for i, v in enumerate(X)})
gdd7 = gdd7.is_resolvable(True)[1] # the relabeled classes
# The first parallel class contains 01(n'-1), the second contains
# 23(n'-1), etc..
# Then remove the blocks containing (n'-1)
for B in gdd4:
for i, b in enumerate(B):
if 8 in b:
j = min(b)
del B[i]
B.insert(0, j)
break
gdd4.sort()
for B in gdd4:
B.pop(0)
for B in gdd7:
for i, b in enumerate(B):
if 14 in b:
j = min(b)
del B[i]
B.insert(0, j)
break
gdd7.sort()
for B in gdd7:
B.pop(0)
# Pasting the KTS(n') without {x,x',\infty} blocks
classes = [[] for i in range((v - 1) // 2)]
gdd = {4: gdd4, 7: gdd7}
for B in PBD_4_7((v - 1) // 2, check=False):
for i, classs in enumerate(gdd[len(B)]):
classes[B[i]].extend([[2 * B[x // 2] + x % 2 for x in BB]
for BB in classs])
# The {x,x',\infty} blocks
for i, classs in enumerate(classes):
classs.append([2 * i, 2 * i + 1, v - 1])
KTS = BalancedIncompleteBlockDesign(
v,
blocks=[tr for cl in classes for tr in cl],
k=3,
lambd=1,
check=True,
copy=False)
KTS._classes = classes
assert KTS.is_resolvable()
return KTS
def T2starGeneralizedQuadrangleGraph(q, dual=False, hyperoval=None, field=None, check_hyperoval=True):
r"""
Return the collinearity graph of the generalized quadrangle `T_2^*(q)`, or of its dual
Let `q=2^k` and `\Theta=PG(3,q)`. `T_2^*(q)` is a generalized quadrangle [GQwiki]_
of order `(q-1,q+1)`, see 3.1.3 in [PT09]_. Fix a plane `\Pi \subset \Theta` and a
`hyperoval <http://en.wikipedia.org/wiki/Oval_(projective_plane)#Even_q>`__
`O \subset \Pi`. The points of `T_2^*(q):=T_2^*(O)` are the points of `\Theta`
outside `\Pi`, and the lines are the lines of `\Theta` outside `\Pi`
that meet `\Pi` in a point of `O`.
INPUT:
- ``q`` -- a power of two
- ``dual`` -- if ``False`` (default), return the graph of `T_2^*(O)`.
Otherwise return the graph of the dual `T_2^*(O)`.
- ``hyperoval`` -- a hyperoval (i.e. a complete 2-arc; a set of points in the plane
meeting every line in 0 or 2 points) in the plane of points with 0th coordinate
0 in `PG(3,q)` over the field ``field``. Each point of ``hyperoval`` must be a length 4
vector over ``field`` with 1st non-0 coordinate equal to 1. By default, ``hyperoval`` and
``field`` are not specified, and constructed on the fly. In particular, ``hyperoval``
we build is the classical one, i.e. a conic with the point of intersection of its
tangent lines.
- ``field`` -- an instance of a finite field of order `q`, must be provided
if ``hyperoval`` is provided.
- ``check_hyperoval`` -- (default: ``True``) if ``True``,
check ``hyperoval`` for correctness.
EXAMPLES:
using the built-in construction::
sage: g=graphs.T2starGeneralizedQuadrangleGraph(4); g
T2*(O,4); GQ(3, 5): Graph on 64 vertices
sage: g.is_strongly_regular(parameters=True)
(64, 18, 2, 6)
sage: g=graphs.T2starGeneralizedQuadrangleGraph(4,dual=True); g
T2*(O,4)*; GQ(5, 3): Graph on 96 vertices
sage: g.is_strongly_regular(parameters=True)
(96, 20, 4, 4)
supplying your own hyperoval::
sage: F=GF(4,'b')
sage: O=[vector(F,(0,0,0,1)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: g=graphs.T2starGeneralizedQuadrangleGraph(4, hyperoval=O, field=F); g
T2*(O,4); GQ(3, 5): Graph on 64 vertices
sage: g.is_strongly_regular(parameters=True)
(64, 18, 2, 6)
TESTS::
sage: F=GF(4,'b') # repeating a point...
sage: O=[vector(F,(0,1,0,0)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: graphs.T2starGeneralizedQuadrangleGraph(4, hyperoval=O, field=F)
Traceback (most recent call last):
...
RuntimeError: incorrect hyperoval size
sage: O=[vector(F,(0,1,1,0)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: graphs.T2starGeneralizedQuadrangleGraph(4, hyperoval=O, field=F)
Traceback (most recent call last):
...
RuntimeError: incorrect hyperoval
"""
from sage.combinat.designs.incidence_structures import IncidenceStructure
from sage.combinat.designs.block_design import ProjectiveGeometryDesign as PG
from sage.modules.free_module_element import free_module_element as vector
p, k = is_prime_power(q,get_data=True)
if k==0 or p!=2:
raise ValueError('q must be a power of 2')
if field is None:
F = FiniteField(q, 'a')
else:
F = field
Theta = PG(3, 1, F, point_coordinates=1)
Pi = set(filter(lambda x: x[0]==F.zero(), Theta.ground_set()))
if hyperoval is None:
O = filter(lambda x: x[1]+x[2]*x[3]==0 or (x[1]==1 and x[2]==0 and x[3]==0), Pi)
O = set(O)
else:
map(lambda x: x.set_immutable(), hyperoval)
O = set(hyperoval)
if check_hyperoval:
if len(O) != q+2:
raise RuntimeError("incorrect hyperoval size")
for L in Theta.blocks():
if set(L).issubset(Pi):
if not len(O.intersection(L)) in [0,2]:
raise RuntimeError("incorrect hyperoval")
L = map(lambda z: filter(lambda y: not y in O, z),
filter(lambda x: len(O.intersection(x)) == 1, Theta.blocks()))
if dual:
G = IncidenceStructure(L).intersection_graph()
G.name('T2*(O,'+str(q)+')*; GQ'+str((q+1,q-1)))
else:
G = IncidenceStructure(L).dual().intersection_graph()
G.name('T2*(O,'+str(q)+'); GQ'+str((q-1,q+1)))
return G
def difference_matrix(g, k, lmbda=1, existence=False, check=True):
r"""
Return a `(g,k,\lambda)`-difference matrix
A matrix `M` is a `(g,k,\lambda)`-difference matrix if it has size `\lambda
g\times k`, its entries belong to the group `G` of cardinality `g`, and
for any two rows `R,R'` of `M` and `x\in G` there are exactly `\lambda`
values `i` such that `R_i-R'_i=x`.
INPUT:
- ``k`` -- (integer) number of columns. If ``k=None`` it is set to the
largest value available.
- ``g`` -- (integer) cardinality of the group `G`
- ``lmbda`` -- (integer; default: 1) -- number of times each element of `G`
appears as a difference.
- ``check`` -- (boolean) Whether to check that output is correct before
returning it. As this is expected to be useless (but we are cautious
guys), you may want to disable it whenever you want speed. Set to
``True`` by default.
- ``existence`` (boolean) -- instead of building the design, return:
- ``True`` -- meaning that Sage knows how to build the design
- ``Unknown`` -- meaning that Sage does not know how to build the
design, but that the design may exist (see :mod:`sage.misc.unknown`).
- ``False`` -- meaning that the design does not exist.
.. NOTE::
When ``k=None`` and ``existence=True`` the function returns an
integer, i.e. the largest `k` such that we can build a
`(g,k,\lambda)`-DM.
EXAMPLES::
sage: G,M = designs.difference_matrix(25,10); G
Finite Field in x of size 5^2
sage: designs.difference_matrix(993,None,existence=1)
32
Here we print for each `g` the maximum possible `k` for which Sage knows
how to build a `(g,k,1)`-difference matrix::
sage: for g in range(2,30):
....: k_max = designs.difference_matrix(g=g,k=None,existence=True)
....: print "{:2} {}".format(g, k_max)
....: _ = designs.difference_matrix(g,k_max)
2 2
3 3
4 4
5 5
6 2
7 7
8 8
9 9
10 2
11 11
12 6
13 13
14 2
15 3
16 16
17 17
18 2
19 19
20 4
21 6
22 2
23 23
24 8
25 25
26 2
27 27
28 6
29 29
TESTS::
sage: designs.difference_matrix(10,12,1,existence=True)
False
sage: designs.difference_matrix(10,12,1)
Traceback (most recent call last):
...
EmptySetError: No (10,12,1)-Difference Matrix exists as k(=12)>g(=10)
sage: designs.difference_matrix(10,9,1,existence=True)
Unknown
sage: designs.difference_matrix(10,9,1)
Traceback (most recent call last):
...
NotImplementedError: I don't know how to build a (10,9,1)-Difference Matrix!
"""
if lmbda == 1 and k is not None and k > g:
if existence:
return False
raise EmptySetError("No ({},{},{})-Difference Matrix exists as k(={})>g(={})".format(g, k, lmbda, k, g))
# Prime powers
elif lmbda == 1 and is_prime_power(g):
if k is None:
if existence:
return g
else:
k = g
elif existence:
return True
F = FiniteField(g, "x")
F_set = list(F)
F_k_set = F_set[:k]
G = F
M = [[x * y for y in F_k_set] for x in F_set]
# Treat the case k=None
# (find the max k such that there exists a DM)
elif k is None:
i = 2
while difference_matrix(g=g, k=i, lmbda=lmbda, existence=True):
i += 1
return i - 1
# From the database
elif (g, lmbda) in DM_constructions and DM_constructions[g, lmbda][0] >= k:
if existence:
return True
_, f = DM_constructions[g, lmbda]
G, M = f()
M = [R[:k] for R in M]
# Product construction
elif find_product_decomposition(g, k, lmbda):
if existence:
return True
(g1, lmbda1), (g2, lmbda2) = find_product_decomposition(g, k, lmbda)
G1, M1 = difference_matrix(g1, k, lmbda1)
G2, M2 = difference_matrix(g2, k, lmbda2)
G, M = difference_matrix_product(k, M1, G1, lmbda1, M2, G2, lmbda2, check=False)
else:
if existence:
return Unknown
raise NotImplementedError("I don't know how to build a ({},{},{})-Difference Matrix!".format(g, k, lmbda))
if check and not is_difference_matrix(M, G, k, lmbda, 1):
raise RuntimeError("Sage built something which is not a ({},{},{})-DM!".format(g, k, lmbda))
return G, M
def TaylorTwographDescendantSRG(q, clique_partition=None):
r"""
constructing the descendant graph of the Taylor's two-graph for `U_3(q)`, `q` odd
This is a strongly regular graph with parameters
`(v,k,\lambda,\mu)=(q^3, (q^2+1)(q-1)/2, (q-1)^3/4-1, (q^2+1)(q-1)/4)`
obtained as a two-graph descendant of the
:func:`Taylor's two-graph <sage.combinat.designs.twographs.taylor_twograph>` `T`.
This graph admits a partition into cliques of size `q`, which are useful in
:func:`TaylorTwographSRG <sage.graphs.generators.classical_geometries.TaylorTwographSRG>`,
a strongly regular graph on `q^3+1` vertices in the
Seidel switching class of `T`, for which we need `(q^2+1)/2` cliques.
The cliques are the `q^2` lines on `v_0` of the projective plane containing the unital
for `U_3(q)`, and intersecting the unital (i.e. the vertices of the graph and the point
we remove) in `q+1` points. This is all taken from §7E of [BvL84]_.
INPUT:
- ``q`` -- a power of an odd prime number
- ``clique_partition`` -- if ``True``, return `q^2-1` cliques of size `q`
with empty pairwise intersection. (Removing all of them leaves a clique, too),
and the point removed from the unital.
EXAMPLES::
sage: g=graphs.TaylorTwographDescendantSRG(3); g
Taylor two-graph descendant SRG: Graph on 27 vertices
sage: g.is_strongly_regular(parameters=True)
(27, 10, 1, 5)
sage: from sage.combinat.designs.twographs import taylor_twograph
sage: T = taylor_twograph(3) # long time
sage: g.is_isomorphic(T.descendant(T.ground_set()[1])) # long time
True
sage: g=graphs.TaylorTwographDescendantSRG(5) # not tested (long time)
sage: g.is_strongly_regular(parameters=True) # not tested (long time)
(125, 52, 15, 26)
TESTS::
sage: g,l,_=graphs.TaylorTwographDescendantSRG(3,clique_partition=True)
sage: all(map(lambda x: g.is_clique(x), l))
True
sage: graphs.TaylorTwographDescendantSRG(4)
Traceback (most recent call last):
...
ValueError: q must be an odd prime power
sage: graphs.TaylorTwographDescendantSRG(6)
Traceback (most recent call last):
...
ValueError: q must be an odd prime power
"""
p, k = is_prime_power(q,get_data=True)
if k==0 or p==2:
raise ValueError('q must be an odd prime power')
from sage.schemes.projective.projective_space import ProjectiveSpace
from sage.modules.free_module_element import free_module_element as vector
from sage.rings.finite_rings.integer_mod import mod
from __builtin__ import sum
Fq = FiniteField(q**2, 'a')
PG = map(tuple,ProjectiveSpace(2, Fq))
def S(x,y):
return sum(map(lambda j: x[j]*y[2-j]**q, xrange(3)))
V = filter(lambda x: S(x,x)==0, PG) # the points of the unital
v0 = V[0]
V.remove(v0)
if mod(q,4)==1:
G = Graph([V,lambda y,z: not (S(v0,y)*S(y,z)*S(z,v0)).is_square()], loops=False)
else:
G = Graph([V,lambda y,z: (S(v0,y)*S(y,z)*S(z,v0)).is_square()], loops=False)
G.name("Taylor two-graph descendant SRG")
if clique_partition:
lines = map(lambda x: filter(lambda t: t[0]+x*t[1]==0, V),
filter(lambda z: z != 0, Fq))
return (G, lines, v0)
else:
return G
def AhrensSzekeresGeneralizedQuadrangleGraph(q, dual=False):
r"""
Return the collinearity graph of the generalized quadrangle `AS(q)`, or of its dual
Let `q` be an odd prime power. `AS(q)` is a generalized quadrangle [GQwiki]_ of
order `(q-1,q+1)`, see 3.1.5 in [PT09]_. Its points are elements
of `F_q^3`, and lines are sets of size `q` of the form
* `\{ (\sigma, a, b) \mid \sigma\in F_q \}`
* `\{ (a, \sigma, b) \mid \sigma\in F_q \}`
* `\{ (c \sigma^2 - b \sigma + a, -2 c \sigma + b, \sigma) \mid \sigma\in F_q \}`,
where `a`, `b`, `c` are arbitrary elements of `F_q`.
INPUT:
- ``q`` -- a power of an odd prime number
- ``dual`` -- if ``False`` (default), return the collinearity graph of `AS(q)`.
Otherwise return the collinearity graph of the dual `AS(q)`.
EXAMPLES::
sage: g=graphs.AhrensSzekeresGeneralizedQuadrangleGraph(5); g
AS(5); GQ(4, 6): Graph on 125 vertices
sage: g.is_strongly_regular(parameters=True)
(125, 28, 3, 7)
sage: g=graphs.AhrensSzekeresGeneralizedQuadrangleGraph(5,dual=True); g
AS(5)*; GQ(6, 4): Graph on 175 vertices
sage: g.is_strongly_regular(parameters=True)
(175, 30, 5, 5)
REFERENCE:
.. [GQwiki] `Generalized quadrangle
<http://en.wikipedia.org/wiki/Generalized_quadrangle>`__
.. [PT09] S. Payne, J. A. Thas.
Finite generalized quadrangles.
European Mathematical Society,
2nd edition, 2009.
"""
from sage.combinat.designs.incidence_structures import IncidenceStructure
p, k = is_prime_power(q,get_data=True)
if k==0 or p==2:
raise ValueError('q must be an odd prime power')
F = FiniteField(q, 'a')
L = []
for a in F:
for b in F:
L.append(tuple(map(lambda s: (s, a, b), F)))
L.append(tuple(map(lambda s: (a, s, b), F)))
for c in F:
L.append(tuple(map(lambda s: (c*s**2 - b*s + a, -2*c*s + b, s), F)))
if dual:
G = IncidenceStructure(L).intersection_graph()
G.name('AS('+str(q)+')*; GQ'+str((q+1,q-1)))
else:
G = IncidenceStructure(L).dual().intersection_graph()
G.name('AS('+str(q)+'); GQ'+str((q-1,q+1)))
return G
def NonisotropicUnitaryPolarGraph(m, q):
r"""
Returns the Graph `NU(m,q)`.
Returns the graph on nonisotropic, with respect to a nondegenerate
Hermitean form, points of the `(m-1)`-dimensional projective space over `F_q`,
with points adjacent whenever they lie on a tangent (to the set of isotropic points)
line.
For more information, see Sect. 9.9 of [BH12]_ and series C14 in [Hu75]_.
INPUT:
- ``m,q`` (integers) -- `q` must be a prime power.
EXAMPLES::
sage: g=graphs.NonisotropicUnitaryPolarGraph(5,2); g
NU(5, 2): Graph on 176 vertices
sage: g.is_strongly_regular(parameters=True)
(176, 135, 102, 108)
TESTS::
sage: graphs.NonisotropicUnitaryPolarGraph(4,2).is_strongly_regular(parameters=True)
(40, 27, 18, 18)
sage: graphs.NonisotropicUnitaryPolarGraph(4,3).is_strongly_regular(parameters=True) # long time
(540, 224, 88, 96)
sage: graphs.NonisotropicUnitaryPolarGraph(6,6)
Traceback (most recent call last):
...
ValueError: q must be a prime power
REFERENCE:
.. [Hu75] X. L. Hubaut.
Strongly regular graphs.
Disc. Math. 13(1975), pp 357--381.
http://dx.doi.org/10.1016/0012-365X(75)90057-6
"""
p, k = is_prime_power(q,get_data=True)
if k==0:
raise ValueError('q must be a prime power')
from sage.libs.gap.libgap import libgap
from itertools import combinations
F=libgap.GF(q**2) # F_{q^2}
W=libgap.FullRowSpace(F, m) # F_{q^2}^m
B=libgap.Elements(libgap.Basis(W)) # the standard basis of W
if m % 2 != 0:
point = B[(m-1)/2]
else:
if p==2:
point = B[m/2] + F.PrimitiveRoot()*B[(m-2)/2]
else:
point = B[(m-2)/2] + B[m/2]
g = libgap.GeneralUnitaryGroup(m,q)
V = libgap.Orbit(g,point,libgap.OnLines) # orbit on nonisotropic points
gp = libgap.Action(g,V,libgap.OnLines) # make a permutation group
s = libgap.Subspace(W,[point, point+B[0]]) # a tangent line on point
# and the points there
sp = [libgap.Elements(libgap.Basis(x))[0] for x in libgap.Elements(s.Subspaces(1))]
h = libgap.Set(map(lambda x: libgap.Position(V, x), libgap.Intersection(V,sp))) # indices
L = libgap.Orbit(gp, h, libgap.OnSets) # orbit on the tangent lines
G = Graph()
for x in L: # every pair of points in the subspace is adjacent to each other in G
G.add_edges(combinations(x, 2))
G.relabel()
G.name("NU" + str((m, q)))
return G
def NonisotropicOrthogonalPolarGraph(m, q, sign="+", perp=None):
r"""
Returns the Graph `NO^{\epsilon,\perp}_{m}(q)`
Let the vectorspace of dimension `m` over `F_q` be
endowed with a nondegenerate quadratic form `F`, of type ``sign`` for `m` even.
* `m` even: assume further that `q=2` or `3`. Returns the graph of the
points (in the underlying projective space) `x` satisfying `F(x)=1`, with adjacency
given by orthogonality w.r.t. `F`. Parameter ``perp`` is ignored.
* `m` odd: if ``perp`` is not ``None``, then we assume that `q=5` and
return the graph of the points `x` satisfying `F(x)=\pm 1` if ``sign="+"``,
respectively `F(x) \in \{2,3\}` if ``sign="-"``, with adjacency
given by orthogonality w.r.t. `F` (cf. Sect 7.D of [BvL84]_).
Otherwise return the graph
of nongenerate hyperplanes of type ``sign``, adjacent whenever the intersection
is degenerate (cf. Sect. 7.C of [BvL84]_).
Note that for `q=2` one will get a complete graph.
For more information, see Sect. 9.9 of [BH12]_ and [BvL84]_. Note that the `page of
Andries Brouwer's website <http://www.win.tue.nl/~aeb/graphs/srghub.html>`_
uses different notation.
INPUT:
- ``m`` - integer, half the dimension of the underlying vectorspace
- ``q`` - a power of a prime number, the size of the underlying field
- ``sign`` -- ``"+"`` (default) or ``"-"``.
EXAMPLES:
`NO^-(4,2)` is isomorphic to Petersen graph::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(4,2,'-'); g
NO^-(4, 2): Graph on 10 vertices
sage: g.is_strongly_regular(parameters=True)
(10, 3, 0, 1)
`NO^-(6,2)` and `NO^+(6,2)`::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,2,'-')
sage: g.is_strongly_regular(parameters=True)
(36, 15, 6, 6)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,2,'+'); g
NO^+(6, 2): Graph on 28 vertices
sage: g.is_strongly_regular(parameters=True)
(28, 15, 6, 10)
`NO^+(8,2)`::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(8,2,'+')
sage: g.is_strongly_regular(parameters=True)
(120, 63, 30, 36)
Wilbrink's graphs for `q=5`::
sage: graphs.NonisotropicOrthogonalPolarGraph(5,5,perp=1).is_strongly_regular(parameters=True) # long time
(325, 60, 15, 10)
sage: graphs.NonisotropicOrthogonalPolarGraph(5,5,'-',perp=1).is_strongly_regular(parameters=True) # long time
(300, 65, 10, 15)
Wilbrink's graphs::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(5,4,'+')
sage: g.is_strongly_regular(parameters=True)
(136, 75, 42, 40)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(5,4,'-')
sage: g.is_strongly_regular(parameters=True)
(120, 51, 18, 24)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(7,4,'+'); g # not tested (long time)
NO^+(7, 4): Graph on 2080 vertices
sage: g.is_strongly_regular(parameters=True) # not tested (long time)
(2080, 1071, 558, 544)
TESTS::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(4,2); g
NO^+(4, 2): Graph on 6 vertices
sage: graphs.NonisotropicOrthogonalPolarGraph(4,3,'-').is_strongly_regular(parameters=True)
(15, 6, 1, 3)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(3,5,'-',perp=1); g
NO^-,perp(3, 5): Graph on 10 vertices
sage: g.is_strongly_regular(parameters=True)
(10, 3, 0, 1)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,3,'+') # long time
sage: g.is_strongly_regular(parameters=True) # long time
(117, 36, 15, 9)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,3,'-'); g # long time
NO^-(6, 3): Graph on 126 vertices
sage: g.is_strongly_regular(parameters=True) # long time
(126, 45, 12, 18)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(5,5,'-') # long time
sage: g.is_strongly_regular(parameters=True) # long time
(300, 104, 28, 40)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(5,5,'+') # long time
sage: g.is_strongly_regular(parameters=True) # long time
(325, 144, 68, 60)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,4,'+')
Traceback (most recent call last):
...
ValueError: for m even q must be 2 or 3
"""
from sage.graphs.generators.classical_geometries import _orthogonal_polar_graph
p, k = is_prime_power(q,get_data=True)
if k==0:
raise ValueError('q must be a prime power')
dec = ''
if m % 2 == 0:
if q in [2,3]:
G = _orthogonal_polar_graph(m, q, sign=sign, point_type=[1])
else:
raise ValueError("for m even q must be 2 or 3")
elif not perp is None:
if q == 5:
G = _orthogonal_polar_graph(m, q, point_type=\
[-1,1] if sign=='+' else [2,3] if sign=='-' else [])
dec = ",perp"
else:
raise ValueError("for perp not None q must be 5")
else:
if not sign in ['+','-']:
raise ValueError("sign must be '+' or '-'")
from sage.libs.gap.libgap import libgap
g0 = libgap.GeneralOrthogonalGroup(m,q)
g = libgap.Group(libgap.List(g0.GeneratorsOfGroup(),libgap.TransposedMat))
F=libgap.GF(q) # F_q
W=libgap.FullRowSpace(F, m) # F_q^m
e = 1 if sign=='+' else -1
n = (m-1)/2
# we build (q^n(q^n+e)/2, (q^n-e)(q^(n-1)+e), 2(q^(2n-2)-1)+eq^(n-1)(q-1),
# 2q^(n-1)(q^(n-1)+e))-srg
# **use** v and k to select appropriate orbit and orbital
nvert = (q**n)*(q**n+e)/2 # v
deg = (q**n-e)*(q**(n-1)+e) # k
S=map(lambda x: libgap.Elements(libgap.Basis(x))[0], \
libgap.Elements(libgap.Subspaces(W,1)))
V = filter(lambda x: len(x)==nvert, libgap.Orbits(g,S,libgap.OnLines))
assert len(V)==1
V = V[0]
gp = libgap.Action(g,V,libgap.OnLines) # make a permutation group
h = libgap.Stabilizer(gp,1)
Vh = filter(lambda x: len(x)==deg, libgap.Orbits(h,libgap.Orbit(gp,1)))
assert len(Vh)==1
Vh = Vh[0][0]
L = libgap.Orbit(gp, [1, Vh], libgap.OnSets)
G = Graph()
G.add_edges(L)
G.name("NO^" + sign + dec + str((m, q)))
return G
def CossidentePenttilaGraph(q):
r"""
Cossidente-Penttila `((q^3+1)(q+1)/2,(q^2+1)(q-1)/2,(q-3)/2,(q-1)^2/2)`-strongly regular graph
For each odd prime power `q`, one can partition the points of the `O_6^-(q)`-generalized
quadrange `GQ(q,q^2)` into two parts, so that on any of them the induced subgraph of
the point graph of the GQ has parameters as above [CP05]_.
Directly follwing the construction in [CP05]_ is not efficient,
as one then needs to construct the dual `GQ(q^2,q)`. Thus we
describe here a more efficient approach that we came up with, following a suggestion by
T.Penttila. Namely, this partition is invariant
under the subgroup `H=\Omega_3(q^2)<O_6^-(q)`. We build the appropriate `H`, which
leaves the form `B(X,Y,Z)=XY+Z^2` invariant, and
pick up two orbits of `H` on the `F_q`-points. One them is `B`-isotropic, and we
take the representative `(1:0:0)`. The other one corresponds to the points of
`PG(2,q^2)` that have all the lines on them either missing the conic specified by `B`, or
intersecting the conic in two points. We take `(1:1:e)` as the representative. It suffices
to pick `e` so that `e^2+1` is not a square in `F_{q^2}`. Indeed,
The conic can be viewed as the union of `\{(0:1:0)\}` and `\{(1:-t^2:t) | t \in F_{q^2}\}`.
The coefficients of a generic line on `(1:1:e)` are `[1:-1-eb:b]`, for `-1\neq eb`.
Thus, to make sure the intersection with the conic is always even, we need that the
discriminant of `1+(1+eb)t^2+tb=0` never vanishes, and this is if and only if
`e^2+1` is not a square. Further, we need to adjust `B`, by multiplying it by appropriately
chosen `\nu`, so that `(1:1:e)` becomes isotropic under the relative trace norm
`\nu B(X,Y,Z)+(\nu B(X,Y,Z))^q`. The latter is used then to define the graph.
INPUT:
- ``q`` -- an odd prime power.
EXAMPLES:
For `q=3` one gets Sims-Gewirtz graph. ::
sage: G=graphs.CossidentePenttilaGraph(3) # optional - gap_packages (grape)
sage: G.is_strongly_regular(parameters=True) # optional - gap_packages (grape)
(56, 10, 0, 2)
For `q>3` one gets new graphs. ::
sage: G=graphs.CossidentePenttilaGraph(5) # optional - gap_packages (grape)
sage: G.is_strongly_regular(parameters=True) # optional - gap_packages (grape)
(378, 52, 1, 8)
TESTS::
sage: G=graphs.CossidentePenttilaGraph(7) # optional - gap_packages (grape) # long time
sage: G.is_strongly_regular(parameters=True) # optional - gap_packages (grape) # long time
(1376, 150, 2, 18)
sage: graphs.CossidentePenttilaGraph(2)
Traceback (most recent call last):
...
ValueError: q(=2) must be an odd prime power
REFERENCES:
.. [CP05] A.Cossidente and T.Penttila
Hemisystems on the Hermitian surface
Journal of London Math. Soc. 72(2005), 731-741
"""
p, k = is_prime_power(q,get_data=True)
if k==0 or p==2:
raise ValueError('q(={}) must be an odd prime power'.format(q))
from sage.libs.gap.libgap import libgap
from sage.misc.package import is_package_installed, PackageNotFoundError
if not is_package_installed('gap_packages'):
raise PackageNotFoundError('gap_packages')
adj_list=libgap.function_factory("""function(q)
local z, e, so, G, nu, G1, G0, B, T, s, O1, O2, x;
LoadPackage("grape");
G0:=SO(3,q^2);
so:=GeneratorsOfGroup(G0);
G1:=Group(Comm(so[1],so[2]),Comm(so[1],so[3]),Comm(so[2],so[3]));
B:=InvariantBilinearForm(G0).matrix;
z:=Z(q^2); e:=z; sqo:=(q^2-1)/2;
if IsInt(sqo/Order(e^2+z^0)) then
e:=z^First([2..q^2-2], x-> not IsInt(sqo/Order(z^(2*x)+z^0)));
fi;
nu:=z^First([0..q^2-2], x->z^x*(e^2+z^0)+(z^x*(e^2+z^0))^q=0*z);
T:=function(x)
local r;
r:=nu*x*B*x;
return r+r^q;
end;
s:=Group([Z(q)*IdentityMat(3,GF(q))]);
O1:=Orbit(G1, Set(Orbit(s,z^0*[1,0,0])), OnSets);
O2:=Orbit(G1, Set(Orbit(s,z^0*[1,1,e])), OnSets);
G:=Graph(G1,Concatenation(O1,O2),OnSets,
function(x,y) return x<>y and 0*z=T(x[1]+y[1]); end);
return List([1..OrderGraph(G)],x->Adjacency(G,x));
end;""")
adj = adj_list(q) # for each vertex, we get the list of vertices it is adjacent to
G = Graph(((i,int(j-1))
for i,ni in enumerate(adj) for j in ni),
format='list_of_edges', multiedges=False)
G.name('CossidentePenttila('+str(q)+')')
return G
def HaemersGraph(q, hyperoval=None, hyperoval_matching=None, field=None, check_hyperoval=True):
r"""
Return the Haemers graph obtained from `T_2^*(q)^*`
Let `q` be a power of 2. In Sect. 8.A of [BvL84]_ one finds a construction
of a strongly regular graph with parameters `(q^2(q+2),q^2+q-1,q-2,q)` from
the graph of `T_2^*(q)^*`, constructed by
:func:`~sage.graphs.graph_generators.GraphGenerators.T2starGeneralizedQuadrangleGraph`,
by redefining adjacencies in the way specified by an arbitrary ``hyperoval_matching``
of the the points (i.e. partitioning into size two parts) of ``hyperoval`` defining
`T_2^*(q)^*`.
While [BvL84]_ gives the construction in geometric terms, it can be formulated,
and is implemented, in graph-theoretic ones, of re-adjusting the edges.
Namely, `G=T_2^*(q)^*` has a partition
into `q+2` independent sets `I_k` of size `q^2` each. Each vertex in `I_j` is
adajcent to `q` vertices from `I_k`. Each `I_k` is paired to some `I_{k'}`,
according to ``hyperoval_matching``. One adds edges `(s,t)` for `s,t \in I_k` whenever
`s` and `t` are adjacent to some `u \in I_{k'}`, and removes all the edges
between `I_k` and `I_{k'}`.
INPUT:
- ``q`` -- a power of two
- ``hyperoval_matching`` -- if ``None`` (default), pair each `i`-th point of
``hyperoval`` with `(i+1)`-th. Otherwise, specifies the pairing
in the format `((i_1,i'_1),(i_2,i'_2),...)`.
- ``hyperoval`` -- a hyperoval defining `T_2^*(q)^*`. If ``None`` (default),
the classical hyperoval obtained from a conic is used. See the
documentation of
:func:`~sage.graphs.graph_generators.GraphGenerators.T2starGeneralizedQuadrangleGraph`,
for more information.
- ``field`` -- an instance of a finite field of order `q`, must be provided
if ``hyperoval`` is provided.
- ``check_hyperoval`` -- (default: ``True``) if ``True``, check
``hyperoval`` for correctness.
EXAMPLES:
using the built-in constructions::
sage: g=graphs.HaemersGraph(4); g
Haemers(4): Graph on 96 vertices
sage: g.is_strongly_regular(parameters=True)
(96, 19, 2, 4)
supplying your own hyperoval_matching::
sage: g=graphs.HaemersGraph(4,hyperoval_matching=((0,5),(1,4),(2,3))); g
Haemers(4): Graph on 96 vertices
sage: g.is_strongly_regular(parameters=True)
(96, 19, 2, 4)
TESTS::
sage: F=GF(4,'b') # repeating a point...
sage: O=[vector(F,(0,1,0,0)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: graphs.HaemersGraph(4, hyperoval=O, field=F)
Traceback (most recent call last):
...
RuntimeError: incorrect hyperoval size
sage: O=[vector(F,(0,1,1,0)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: graphs.HaemersGraph(4, hyperoval=O, field=F)
Traceback (most recent call last):
...
RuntimeError: incorrect hyperoval
sage: g=graphs.HaemersGraph(8); g # not tested (long time)
Haemers(8): Graph on 640 vertices
sage: g.is_strongly_regular(parameters=True) # not tested (long time)
(640, 71, 6, 8)
"""
from sage.modules.free_module_element import free_module_element as vector
from sage.rings.finite_rings.finite_field_constructor import GF
from itertools import combinations
p, k = is_prime_power(q,get_data=True)
if k==0 or p!=2:
raise ValueError('q must be a power of 2')
if hyperoval_matching is None:
hyperoval_matching = map(lambda k: (2*k+1,2*k), xrange(1+q/2))
if field is None:
F = GF(q,'a')
else:
F = field
# for q=8, 95% of CPU time taken by this function is spent in the follwing call
G = T2starGeneralizedQuadrangleGraph(q, field=F, dual=True, hyperoval=hyperoval, check_hyperoval=check_hyperoval)
def normalize(v): # make sure the 1st non-0 coordinate is 1.
d=next(x for x in v if x!=F.zero())
return vector(map(lambda x: x/d, v))
# build the partition into independent sets
P = map(lambda x: normalize(x[0]-x[1]), G.vertices())
O = list(set(map(tuple,P)))
I_ks = {x:[] for x in range(q+2)} # the partition into I_k's
for i in xrange(len(P)):
I_ks[O.index(tuple(P[i]))].append(i)
# perform the adjustment of the edges, as described.
G.relabel()
cliques = []
for i,j in hyperoval_matching:
Pij = set(I_ks[i]+I_ks[j])
for v in Pij:
cliques.append(Pij.intersection(G.neighbors(v)))
G.delete_edges(G.edge_boundary(I_ks[i],I_ks[j])) # edges on (I_i,I_j)
G.add_edges(e for c in cliques for e in combinations(c,2))
G.name('Haemers('+str(q)+')')
return G
def difference_matrix(g, k, lmbda=1, existence=False, check=True):
r"""
Return a `(g,k,\lambda)`-difference matrix
A matrix `M` is a `(g,k,\lambda)`-difference matrix if it has size `\lambda
g\times k`, its entries belong to the group `G` of cardinality `g`, and
for any two rows `R,R'` of `M` and `x\in G` there are exactly `\lambda`
values `i` such that `R_i-R'_i=x`.
INPUT:
- ``k`` -- (integer) number of columns. If ``k=None`` it is set to the
largest value available.
- ``g`` -- (integer) cardinality of the group `G`
- ``lmbda`` -- (integer; default: 1) -- number of times each element of `G`
appears as a difference.
- ``check`` -- (boolean) Whether to check that output is correct before
returning it. As this is expected to be useless (but we are cautious
guys), you may want to disable it whenever you want speed. Set to
``True`` by default.
- ``existence`` (boolean) -- instead of building the design, return:
- ``True`` -- meaning that Sage knows how to build the design
- ``Unknown`` -- meaning that Sage does not know how to build the
design, but that the design may exist (see :mod:`sage.misc.unknown`).
- ``False`` -- meaning that the design does not exist.
.. NOTE::
When ``k=None`` and ``existence=True`` the function returns an
integer, i.e. the largest `k` such that we can build a
`(g,k,\lambda)`-DM.
EXAMPLES::
sage: G,M = designs.difference_matrix(25,10); G
Finite Field in x of size 5^2
sage: designs.difference_matrix(993,None,existence=1)
32
Here we print for each `g` the maximum possible `k` for which Sage knows
how to build a `(g,k,1)`-difference matrix::
sage: for g in range(2,30):
....: k_max = designs.difference_matrix(g=g,k=None,existence=True)
....: print("{:2} {}".format(g, k_max))
....: _ = designs.difference_matrix(g,k_max)
2 2
3 3
4 4
5 5
6 2
7 7
8 8
9 9
10 2
11 11
12 6
13 13
14 2
15 3
16 16
17 17
18 2
19 19
20 4
21 6
22 2
23 23
24 8
25 25
26 2
27 27
28 6
29 29
TESTS::
sage: designs.difference_matrix(10,12,1,existence=True)
False
sage: designs.difference_matrix(10,12,1)
Traceback (most recent call last):
...
EmptySetError: No (10,12,1)-Difference Matrix exists as k(=12)>g(=10)
sage: designs.difference_matrix(10,9,1,existence=True)
Unknown
sage: designs.difference_matrix(10,9,1)
Traceback (most recent call last):
...
NotImplementedError: I don't know how to build a (10,9,1)-Difference Matrix!
"""
if lmbda == 1 and k is not None and k > g:
if existence:
return False
raise EmptySetError(
"No ({},{},{})-Difference Matrix exists as k(={})>g(={})".format(
g, k, lmbda, k, g))
# Prime powers
elif lmbda == 1 and is_prime_power(g):
if k is None:
if existence:
return g
else:
k = g
elif existence:
return True
F = FiniteField(g, 'x')
F_set = list(F)
F_k_set = F_set[:k]
G = F
M = [[x * y for y in F_k_set] for x in F_set]
# Treat the case k=None
# (find the max k such that there exists a DM)
elif k is None:
i = 2
while difference_matrix(g=g, k=i, lmbda=lmbda, existence=True):
i += 1
return i - 1
# From the database
elif (g, lmbda) in DM_constructions and DM_constructions[g, lmbda][0] >= k:
if existence:
return True
_, f = DM_constructions[g, lmbda]
G, M = f()
M = [R[:k] for R in M]
# Product construction
elif find_product_decomposition(g, k, lmbda):
if existence:
return True
(g1, lmbda1), (g2, lmbda2) = find_product_decomposition(g, k, lmbda)
G1, M1 = difference_matrix(g1, k, lmbda1)
G2, M2 = difference_matrix(g2, k, lmbda2)
G, M = difference_matrix_product(k,
M1,
G1,
lmbda1,
M2,
G2,
lmbda2,
check=False)
else:
if existence:
return Unknown
raise NotImplementedError(
"I don't know how to build a ({},{},{})-Difference Matrix!".format(
g, k, lmbda))
if check and not is_difference_matrix(M, G, k, lmbda, 1):
raise RuntimeError(
"Sage built something which is not a ({},{},{})-DM!".format(
g, k, lmbda))
return G, M
def NonisotropicOrthogonalPolarGraph(m, q, sign="+", perp=None):
r"""
Returns the Graph `NO^{\epsilon,\perp}_{m}(q)`
Let the vectorspace of dimension `m` over `F_q` be
endowed with a nondegenerate quadratic form `F`, of type ``sign`` for `m` even.
* `m` even: assume further that `q=2` or `3`. Returns the graph of the
points (in the underlying projective space) `x` satisfying `F(x)=1`, with adjacency
given by orthogonality w.r.t. `F`. Parameter ``perp`` is ignored.
* `m` odd: if ``perp`` is not ``None``, then we assume that `q=5` and
return the graph of the points `x` satisfying `F(x)=\pm 1` if ``sign="+"``,
respectively `F(x) \in \{2,3\}` if ``sign="-"``, with adjacency
given by orthogonality w.r.t. `F` (cf. Sect 7.D of [BvL84]_).
Otherwise return the graph
of nongenerate hyperplanes of type ``sign``, adjacent whenever the intersection
is degenerate (cf. Sect. 7.C of [BvL84]_).
Note that for `q=2` one will get a complete graph.
For more information, see Sect. 9.9 of [BH12]_ and [BvL84]_. Note that the `page of
Andries Brouwer's website <http://www.win.tue.nl/~aeb/graphs/srghub.html>`_
uses different notation.
INPUT:
- ``m`` - integer, half the dimension of the underlying vectorspace
- ``q`` - a power of a prime number, the size of the underlying field
- ``sign`` -- ``"+"`` (default) or ``"-"``.
EXAMPLES:
`NO^-(4,2)` is isomorphic to Petersen graph::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(4,2,'-'); g
NO^-(4, 2): Graph on 10 vertices
sage: g.is_strongly_regular(parameters=True)
(10, 3, 0, 1)
`NO^-(6,2)` and `NO^+(6,2)`::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,2,'-')
sage: g.is_strongly_regular(parameters=True)
(36, 15, 6, 6)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,2,'+'); g
NO^+(6, 2): Graph on 28 vertices
sage: g.is_strongly_regular(parameters=True)
(28, 15, 6, 10)
`NO^+(8,2)`::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(8,2,'+')
sage: g.is_strongly_regular(parameters=True)
(120, 63, 30, 36)
Wilbrink's graphs for `q=5`::
sage: graphs.NonisotropicOrthogonalPolarGraph(5,5,perp=1).is_strongly_regular(parameters=True) # long time
(325, 60, 15, 10)
sage: graphs.NonisotropicOrthogonalPolarGraph(5,5,'-',perp=1).is_strongly_regular(parameters=True) # long time
(300, 65, 10, 15)
Wilbrink's graphs::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(5,4,'+')
sage: g.is_strongly_regular(parameters=True)
(136, 75, 42, 40)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(5,4,'-')
sage: g.is_strongly_regular(parameters=True)
(120, 51, 18, 24)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(7,4,'+'); g # not tested (long time)
NO^+(7, 4): Graph on 2080 vertices
sage: g.is_strongly_regular(parameters=True) # not tested (long time)
(2080, 1071, 558, 544)
TESTS::
sage: g=graphs.NonisotropicOrthogonalPolarGraph(4,2); g
NO^+(4, 2): Graph on 6 vertices
sage: graphs.NonisotropicOrthogonalPolarGraph(4,3,'-').is_strongly_regular(parameters=True)
(15, 6, 1, 3)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(3,5,'-',perp=1); g
NO^-,perp(3, 5): Graph on 10 vertices
sage: g.is_strongly_regular(parameters=True)
(10, 3, 0, 1)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,3,'+') # long time
sage: g.is_strongly_regular(parameters=True) # long time
(117, 36, 15, 9)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,3,'-'); g # long time
NO^-(6, 3): Graph on 126 vertices
sage: g.is_strongly_regular(parameters=True) # long time
(126, 45, 12, 18)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(5,5,'-') # long time
sage: g.is_strongly_regular(parameters=True) # long time
(300, 104, 28, 40)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(5,5,'+') # long time
sage: g.is_strongly_regular(parameters=True) # long time
(325, 144, 68, 60)
sage: g=graphs.NonisotropicOrthogonalPolarGraph(6,4,'+')
Traceback (most recent call last):
...
ValueError: for m even q must be 2 or 3
"""
from sage.graphs.generators.classical_geometries import _orthogonal_polar_graph
p, k = is_prime_power(q,get_data=True)
if k==0:
raise ValueError('q must be a prime power')
dec = ''
if m % 2 == 0:
if q in [2,3]:
G = _orthogonal_polar_graph(m, q, sign=sign, point_type=[1])
else:
raise ValueError("for m even q must be 2 or 3")
elif not perp is None:
if q == 5:
G = _orthogonal_polar_graph(m, q, point_type=\
[-1,1] if sign=='+' else [2,3] if sign=='-' else [])
dec = ",perp"
else:
raise ValueError("for perp not None q must be 5")
else:
if not sign in ['+','-']:
raise ValueError("sign must be '+' or '-'")
from sage.libs.gap.libgap import libgap
g0 = libgap.GeneralOrthogonalGroup(m,q)
g = libgap.Group(libgap.List(g0.GeneratorsOfGroup(),libgap.TransposedMat))
F=libgap.GF(q) # F_q
W=libgap.FullRowSpace(F, m) # F_q^m
e = 1 if sign=='+' else -1
n = (m-1)/2
# we build (q^n(q^n+e)/2, (q^n-e)(q^(n-1)+e), 2(q^(2n-2)-1)+eq^(n-1)(q-1),
# 2q^(n-1)(q^(n-1)+e))-srg
# **use** v and k to select appropriate orbit and orbital
nvert = (q**n)*(q**n+e)/2 # v
deg = (q**n-e)*(q**(n-1)+e) # k
S=map(lambda x: libgap.Elements(libgap.Basis(x))[0], \
libgap.Elements(libgap.Subspaces(W,1)))
V = filter(lambda x: len(x)==nvert, libgap.Orbits(g,S,libgap.OnLines))
assert len(V)==1
V = V[0]
gp = libgap.Action(g,V,libgap.OnLines) # make a permutation group
h = libgap.Stabilizer(gp,1)
Vh = filter(lambda x: len(x)==deg, libgap.Orbits(h,libgap.Orbit(gp,1)))
assert len(Vh)==1
Vh = Vh[0][0]
L = libgap.Orbit(gp, [1, Vh], libgap.OnSets)
G = Graph()
G.add_edges(L)
G.name("NO^" + sign + dec + str((m, q)))
return G
def GDD_4_2(q,existence=False,check=True):
r"""
Return a `(2q,\{4\},\{2\})`-GDD for `q` a prime power with `q\equiv 1\pmod{6}`.
This method implements Lemma VII.5.17 from [BJL99] (p.495).
INPUT:
- ``q`` (integer)
- ``existence`` (boolean) -- instead of building the design, return:
- ``True`` -- meaning that Sage knows how to build the design
- ``Unknown`` -- meaning that Sage does not know how to build the
design, but that the design may exist (see :mod:`sage.misc.unknown`).
- ``False`` -- meaning that the design does not exist.
- ``check`` -- (boolean) Whether to check that output is correct before
returning it. As this is expected to be useless (but we are cautious
guys), you may want to disable it whenever you want speed. Set to ``True``
by default.
EXAMPLE::
sage: from sage.combinat.designs.group_divisible_designs import GDD_4_2
sage: GDD_4_2(7,existence=True)
True
sage: GDD_4_2(7)
Group Divisible Design on 14 points of type 2^7
sage: GDD_4_2(8,existence=True)
Unknown
sage: GDD_4_2(8)
Traceback (most recent call last):
...
NotImplementedError
"""
if q <=1 or q%6 != 1 or not is_prime_power(q):
if existence:
return Unknown
raise NotImplementedError
if existence:
return True
from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
G = GF(q,'x')
w = G.primitive_element()
e = w**((q - 1) // 3)
# A first parallel class is defined. G acts on it, which yields all others.
first_class = [[(0,0),(1,w**i),(1,e*w**i),(1,e*e*w**i)]
for i in range((q - 1) // 6)]
label = {p:i for i,p in enumerate(G)}
classes = [[[2*label[x[1]+g]+(x[0]+j)%2 for x in S]
for S in first_class]
for g in G for j in range(2)]
return GroupDivisibleDesign(2*q,
groups = [[i,i+1] for i in range(0,2*q,2)],
blocks = sum(classes,[]),
K = [4],
G = [2],
check = check,
copy = False)
def T2starGeneralizedQuadrangleGraph(q, dual=False, hyperoval=None, field=None, check_hyperoval=True):
r"""
Return the collinearity graph of the generalized quadrangle `T_2^*(q)`, or of its dual
Let `q=2^k` and `\Theta=PG(3,q)`. `T_2^*(q)` is a generalized quadrangle [GQwiki]_
of order `(q-1,q+1)`, see 3.1.3 in [PT09]_. Fix a plane `\Pi \subset \Theta` and a
`hyperoval <http://en.wikipedia.org/wiki/Oval_(projective_plane)#Even_q>`__
`O \subset \Pi`. The points of `T_2^*(q):=T_2^*(O)` are the points of `\Theta`
outside `\Pi`, and the lines are the lines of `\Theta` outside `\Pi`
that meet `\Pi` in a point of `O`.
INPUT:
- ``q`` -- a power of two
- ``dual`` -- if ``False`` (default), return the graph of `T_2^*(O)`.
Otherwise return the graph of the dual `T_2^*(O)`.
- ``hyperoval`` -- a hyperoval (i.e. a complete 2-arc; a set of points in the plane
meeting every line in 0 or 2 points) in the plane of points with 0th coordinate
0 in `PG(3,q)` over the field ``field``. Each point of ``hyperoval`` must be a length 4
vector over ``field`` with 1st non-0 coordinate equal to 1. By default, ``hyperoval`` and
``field`` are not specified, and constructed on the fly. In particular, ``hyperoval``
we build is the classical one, i.e. a conic with the point of intersection of its
tangent lines.
- ``field`` -- an instance of a finite field of order `q`, must be provided
if ``hyperoval`` is provided.
- ``check_hyperoval`` -- (default: ``True``) if ``True``,
check ``hyperoval`` for correctness.
EXAMPLES:
using the built-in construction::
sage: g=graphs.T2starGeneralizedQuadrangleGraph(4); g
T2*(O,4); GQ(3, 5): Graph on 64 vertices
sage: g.is_strongly_regular(parameters=True)
(64, 18, 2, 6)
sage: g=graphs.T2starGeneralizedQuadrangleGraph(4,dual=True); g
T2*(O,4)*; GQ(5, 3): Graph on 96 vertices
sage: g.is_strongly_regular(parameters=True)
(96, 20, 4, 4)
supplying your own hyperoval::
sage: F=GF(4,'b')
sage: O=[vector(F,(0,0,0,1)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: g=graphs.T2starGeneralizedQuadrangleGraph(4, hyperoval=O, field=F); g
T2*(O,4); GQ(3, 5): Graph on 64 vertices
sage: g.is_strongly_regular(parameters=True)
(64, 18, 2, 6)
TESTS::
sage: F=GF(4,'b') # repeating a point...
sage: O=[vector(F,(0,1,0,0)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: graphs.T2starGeneralizedQuadrangleGraph(4, hyperoval=O, field=F)
Traceback (most recent call last):
...
RuntimeError: incorrect hyperoval size
sage: O=[vector(F,(0,1,1,0)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: graphs.T2starGeneralizedQuadrangleGraph(4, hyperoval=O, field=F)
Traceback (most recent call last):
...
RuntimeError: incorrect hyperoval
"""
from sage.combinat.designs.incidence_structures import IncidenceStructure
from sage.combinat.designs.block_design import ProjectiveGeometryDesign as PG
from sage.modules.free_module_element import free_module_element as vector
p, k = is_prime_power(q,get_data=True)
if k==0 or p!=2:
raise ValueError('q must be a power of 2')
if field is None:
F = FiniteField(q, 'a')
else:
F = field
Theta = PG(3, 1, F, point_coordinates=1)
Pi = set(filter(lambda x: x[0]==F.zero(), Theta.ground_set()))
if hyperoval is None:
O = filter(lambda x: x[1]+x[2]*x[3]==0 or (x[1]==1 and x[2]==0 and x[3]==0), Pi)
O = set(O)
else:
map(lambda x: x.set_immutable(), hyperoval)
O = set(hyperoval)
if check_hyperoval:
if len(O) != q+2:
raise RuntimeError("incorrect hyperoval size")
for L in Theta.blocks():
if set(L).issubset(Pi):
if not len(O.intersection(L)) in [0,2]:
raise RuntimeError("incorrect hyperoval")
L = map(lambda z: filter(lambda y: not y in O, z),
filter(lambda x: len(O.intersection(x)) == 1, Theta.blocks()))
if dual:
G = IncidenceStructure(L).intersection_graph()
G.name('T2*(O,'+str(q)+')*; GQ'+str((q+1,q-1)))
else:
G = IncidenceStructure(L).dual().intersection_graph()
G.name('T2*(O,'+str(q)+'); GQ'+str((q-1,q+1)))
return G
def skew_hadamard_matrix(n, existence=False, skew_normalize=True, check=True):
r"""
Tries to construct a skew Hadamard matrix
A Hadamard matrix `H` is called skew if `H=S-I`, for `I` the identity matrix
and `-S=S^\top`. Currently constructions from Section 14.1 of [Ha83]_ and few
more exotic ones are implemented.
INPUT:
- ``n`` (integer) -- dimension of the matrix
- ``existence`` (boolean) -- whether to build the matrix or merely query if
a construction is available in Sage. When set to ``True``, the function
returns:
- ``True`` -- meaning that Sage knows how to build the matrix
- ``Unknown`` -- meaning that Sage does not know how to build the
matrix, but that the design may exist (see :mod:`sage.misc.unknown`).
- ``False`` -- meaning that the matrix does not exist.
- ``skew_normalize`` (boolean) -- whether to make the 1st row all-one, and
adjust the 1st column accordingly. Set to ``True`` by default.
- ``check`` (boolean) -- whether to check that output is correct before
returning it. As this is expected to be useless (but we are cautious
guys), you may want to disable it whenever you want speed. Set to ``True``
by default.
EXAMPLES::
sage: from sage.combinat.matrices.hadamard_matrix import skew_hadamard_matrix
sage: skew_hadamard_matrix(12).det()
2985984
sage: 12^6
2985984
sage: skew_hadamard_matrix(1)
[1]
sage: skew_hadamard_matrix(2)
[ 1 1]
[-1 1]
TESTS::
sage: skew_hadamard_matrix(10,existence=True)
False
sage: skew_hadamard_matrix(12,existence=True)
True
sage: skew_hadamard_matrix(784,existence=True)
True
sage: skew_hadamard_matrix(10)
Traceback (most recent call last):
...
ValueError: A skew Hadamard matrix of order 10 does not exist
sage: skew_hadamard_matrix(36)
36 x 36 dense matrix over Integer Ring...
sage: skew_hadamard_matrix(36)==skew_hadamard_matrix(36,skew_normalize=False)
False
sage: skew_hadamard_matrix(52)
52 x 52 dense matrix over Integer Ring...
sage: skew_hadamard_matrix(92)
92 x 92 dense matrix over Integer Ring...
sage: skew_hadamard_matrix(816) # long time
816 x 816 dense matrix over Integer Ring...
sage: skew_hadamard_matrix(100)
Traceback (most recent call last):
...
ValueError: A skew Hadamard matrix of order 100 is not yet implemented.
sage: skew_hadamard_matrix(100,existence=True)
Unknown
REFERENCES:
.. [Ha83] \M. Hall,
Combinatorial Theory,
2nd edition,
Wiley, 1983
"""
def true():
_skew_had_cache[n] = True
return True
M = None
if existence and n in _skew_had_cache:
return True
if not (n % 4 == 0) and (n > 2):
if existence:
return False
raise ValueError("A skew Hadamard matrix of order %s does not exist" %
n)
if n == 2:
if existence:
return true()
M = matrix([[1, 1], [-1, 1]])
elif n == 1:
if existence:
return true()
M = matrix([1])
elif is_prime_power(n - 1) and ((n - 1) % 4 == 3):
if existence:
return true()
M = hadamard_matrix_paleyI(n, normalize=False)
elif n % 8 == 0:
if skew_hadamard_matrix(n // 2,
existence=True): # (Lemma 14.1.6 in [Ha83]_)
if existence:
return true()
H = skew_hadamard_matrix(n // 2, check=False)
M = block_matrix([[H, H], [-H.T, H.T]])
else: # try Williamson construction (Lemma 14.1.5 in [Ha83]_)
for d in divisors(n)[2:-2]: # skip 1, 2, n/2, and n
n1 = n // d
if is_prime_power(d - 1) and (d % 4 == 0) and (n1 % 4 == 0)\
and skew_hadamard_matrix(n1,existence=True):
if existence:
return true()
H = skew_hadamard_matrix(n1, check=False) - I(n1)
U = matrix(ZZ, d, lambda i, j: -1 if i==j==0 else\
1 if i==j==1 or (i>1 and j-1==d-i)\
else 0)
A = block_matrix(
[[matrix([0]),
matrix(ZZ, 1, d - 1, [1] * (d - 1))],
[
matrix(ZZ, d - 1, 1, [-1] * (d - 1)),
_helper_payley_matrix(d - 1, zero_position=0)
]]) + I(d)
M = A.tensor_product(I(n1)) + (U * A).tensor_product(H)
break
if M is None: # try Williamson-Goethals-Seidel construction
if GS_skew_hadamard_smallcases(n, existence=True):
if existence:
return true()
M = GS_skew_hadamard_smallcases(n)
else:
if existence:
return Unknown
raise ValueError(
"A skew Hadamard matrix of order %s is not yet implemented." %
n)
if skew_normalize:
dd = diagonal_matrix(M[0])
M = dd * M * dd
if check:
assert is_hadamard_matrix(M, normalized=False, skew=True)
if skew_normalize:
from sage.modules.free_module_element import vector
assert M[0] == vector([1] * n)
_skew_had_cache[n] = True
return M
def HaemersGraph(q, hyperoval=None, hyperoval_matching=None, field=None, check_hyperoval=True):
r"""
Return the Haemers graph obtained from `T_2^*(q)^*`
Let `q` be a power of 2. In Sect. 8.A of [BvL84]_ one finds a construction
of a strongly regular graph with parameters `(q^2(q+2),q^2+q-1,q-2,q)` from
the graph of `T_2^*(q)^*`, constructed by
:func:`~sage.graphs.graph_generators.GraphGenerators.T2starGeneralizedQuadrangleGraph`,
by redefining adjacencies in the way specified by an arbitrary ``hyperoval_matching``
of the the points (i.e. partitioning into size two parts) of ``hyperoval`` defining
`T_2^*(q)^*`.
While [BvL84]_ gives the construction in geometric terms, it can be formulated,
and is implemented, in graph-theoretic ones, of re-adjusting the edges.
Namely, `G=T_2^*(q)^*` has a partition
into `q+2` independent sets `I_k` of size `q^2` each. Each vertex in `I_j` is
adajcent to `q` vertices from `I_k`. Each `I_k` is paired to some `I_{k'}`,
according to ``hyperoval_matching``. One adds edges `(s,t)` for `s,t \in I_k` whenever
`s` and `t` are adjacent to some `u \in I_{k'}`, and removes all the edges
between `I_k` and `I_{k'}`.
INPUT:
- ``q`` -- a power of two
- ``hyperoval_matching`` -- if ``None`` (default), pair each `i`-th point of
``hyperoval`` with `(i+1)`-th. Otherwise, specifies the pairing
in the format `((i_1,i'_1),(i_2,i'_2),...)`.
- ``hyperoval`` -- a hyperoval defining `T_2^*(q)^*`. If ``None`` (default),
the classical hyperoval obtained from a conic is used. See the
documentation of
:func:`~sage.graphs.graph_generators.GraphGenerators.T2starGeneralizedQuadrangleGraph`,
for more information.
- ``field`` -- an instance of a finite field of order `q`, must be provided
if ``hyperoval`` is provided.
- ``check_hyperoval`` -- (default: ``True``) if ``True``, check
``hyperoval`` for correctness.
EXAMPLES:
using the built-in constructions::
sage: g=graphs.HaemersGraph(4); g
Haemers(4): Graph on 96 vertices
sage: g.is_strongly_regular(parameters=True)
(96, 19, 2, 4)
supplying your own hyperoval_matching::
sage: g=graphs.HaemersGraph(4,hyperoval_matching=((0,5),(1,4),(2,3))); g
Haemers(4): Graph on 96 vertices
sage: g.is_strongly_regular(parameters=True)
(96, 19, 2, 4)
TESTS::
sage: F=GF(4,'b') # repeating a point...
sage: O=[vector(F,(0,1,0,0)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: graphs.HaemersGraph(4, hyperoval=O, field=F)
Traceback (most recent call last):
...
RuntimeError: incorrect hyperoval size
sage: O=[vector(F,(0,1,1,0)),vector(F,(0,0,1,0))]+map(lambda x: vector(F, (0,1,x^2,x)),F)
sage: graphs.HaemersGraph(4, hyperoval=O, field=F)
Traceback (most recent call last):
...
RuntimeError: incorrect hyperoval
sage: g=graphs.HaemersGraph(8); g # not tested (long time)
Haemers(8): Graph on 640 vertices
sage: g.is_strongly_regular(parameters=True) # not tested (long time)
(640, 71, 6, 8)
"""
from sage.modules.free_module_element import free_module_element as vector
from sage.rings.finite_rings.constructor import GF
from itertools import combinations
p, k = is_prime_power(q,get_data=True)
if k==0 or p!=2:
raise ValueError('q must be a power of 2')
if hyperoval_matching is None:
hyperoval_matching = map(lambda k: (2*k+1,2*k), xrange(1+q/2))
if field is None:
F = GF(q,'a')
else:
F = field
# for q=8, 95% of CPU time taken by this function is spent in the follwing call
G = T2starGeneralizedQuadrangleGraph(q, field=F, dual=True, hyperoval=hyperoval, check_hyperoval=check_hyperoval)
def normalize(v): # make sure the 1st non-0 coordinate is 1.
d=next(x for x in v if x!=F.zero())
return vector(map(lambda x: x/d, v))
# build the partition into independent sets
P = map(lambda x: normalize(x[0]-x[1]), G.vertices())
O = list(set(map(tuple,P)))
I_ks = {x:[] for x in range(q+2)} # the partition into I_k's
for i in xrange(len(P)):
I_ks[O.index(tuple(P[i]))].append(i)
# perform the adjustment of the edges, as described.
G.relabel()
cliques = []
for i,j in hyperoval_matching:
Pij = set(I_ks[i]+I_ks[j])
for v in Pij:
cliques.append(Pij.intersection(G.neighbors(v)))
G.delete_edges(G.edge_boundary(I_ks[i],I_ks[j])) # edges on (I_i,I_j)
G.add_edges(e for c in cliques for e in combinations(c,2))
G.name('Haemers('+str(q)+')')
return G
def rshcd_from_prime_power_and_conference_matrix(n):
r"""
Return a `((n-1)^2,1)`-RSHCD if `n` is prime power, and symmetric `(n-1)`-conference matrix exists
The construction implemented here is Theorem 16 (and Corollary 17) from [WW72]_.
In [SWW72]_ this construction (Theorem 5.15 and Corollary 5.16)
is reproduced with a typo. Note that [WW72]_ refers to [Sz69]_ for the construction,
provided by :func:`szekeres_difference_set_pair`,
of complementary difference sets, and the latter has a typo.
From a :func:`symmetric_conference_matrix`, we only need the Seidel
adjacency matrix of the underlying strongly regular conference (i.e. Paley
type) graph, which we construct directly.
INPUT:
- ``n`` -- an integer
.. SEEALSO::
:func:`regular_symmetric_hadamard_matrix_with_constant_diagonal`
EXAMPLES:
A 36x36 example ::
sage: from sage.combinat.matrices.hadamard_matrix import rshcd_from_prime_power_and_conference_matrix
sage: from sage.combinat.matrices.hadamard_matrix import is_hadamard_matrix
sage: H = rshcd_from_prime_power_and_conference_matrix(7); H
36 x 36 dense matrix over Integer Ring (use the '.str()' method to see the entries)
sage: H==H.T and is_hadamard_matrix(H) and H.diagonal()==[1]*36 and list(sum(H))==[6]*36
True
Bigger examples, only provided by this construction ::
sage: H = rshcd_from_prime_power_and_conference_matrix(27) # long time
sage: H == H.T and is_hadamard_matrix(H) # long time
True
sage: H.diagonal()==[1]*676 and list(sum(H))==[26]*676 # long time
True
In this example the conference matrix is not Paley, as 45 is not a prime power ::
sage: H = rshcd_from_prime_power_and_conference_matrix(47) # not tested (long time)
REFERENCE:
.. [WW72] \J. Wallis and A.L. Whiteman,
Some classes of Hadamard matrices with constant diagonal,
Bull. Austral. Math. Soc. 7(1972), 233-249
"""
from sage.graphs.strongly_regular_db import strongly_regular_graph as srg
if is_prime_power(n) and 2 == (n - 1) % 4:
try:
M = srg(n - 2, (n - 3) // 2, (n - 7) // 4)
except ValueError:
return
m = (n - 3) // 4
Q, X, Y = szekeres_difference_set_pair(m)
B = typeI_matrix_difference_set(Q, X)
A = -typeI_matrix_difference_set(Q, Y) # must be symmetric
W = M.seidel_adjacency_matrix()
f = J(1, 4 * m + 1)
e = J(1, 2 * m + 1)
JJ = J(2 * m + 1, 2 * m + 1)
II = I(n - 2)
Ib = I(2 * m + 1)
J4m = J(4 * m + 1, 4 * m + 1)
H34 = -(B + Ib).tensor_product(W) + Ib.tensor_product(J4m) + (
Ib - JJ).tensor_product(II)
A_t_W = A.tensor_product(W)
e_t_f = e.tensor_product(f)
H = block_matrix(
[[J(1, 1), f, e_t_f, -e_t_f],
[f.T, J4m,
e.tensor_product(W - II),
e.tensor_product(W + II)],
[
e_t_f.T, (e.T).tensor_product(W - II),
A_t_W + JJ.tensor_product(II), H34
],
[
-e_t_f.T, (e.T).tensor_product(W + II), H34.T,
-A_t_W + JJ.tensor_product(II)
]])
return H
def BIBD_from_arc_in_desarguesian_projective_plane(n,k,existence=False):
r"""
Returns a `(n,k,1)`-BIBD from a maximal arc in a projective plane.
This function implements a construction from Denniston [Denniston69]_, who
describes a maximal :meth:`arc
<sage.combinat.designs.bibd.BalancedIncompleteBlockDesign.arc>` in a
:func:`Desarguesian Projective Plane
<sage.combinat.designs.block_design.DesarguesianProjectivePlaneDesign>` of
order `2^k`. From two powers of two `n,q` with `n<q`, it produces a
`((n-1)(q+1)+1,n,1)`-BIBD.
INPUT:
- ``n,k`` (integers) -- must be powers of two (among other restrictions).
- ``existence`` (boolean) -- whether to return the BIBD obtained through
this construction (default), or to merely indicate with a boolean return
value whether this method *can* build the requested BIBD.
EXAMPLES:
A `(232,8,1)`-BIBD::
sage: from sage.combinat.designs.bibd import BIBD_from_arc_in_desarguesian_projective_plane
sage: from sage.combinat.designs.bibd import BalancedIncompleteBlockDesign
sage: D = BIBD_from_arc_in_desarguesian_projective_plane(232,8)
sage: BalancedIncompleteBlockDesign(232,D)
(232,8,1)-Balanced Incomplete Block Design
A `(120,8,1)`-BIBD::
sage: D = BIBD_from_arc_in_desarguesian_projective_plane(120,8)
sage: BalancedIncompleteBlockDesign(120,D)
(120,8,1)-Balanced Incomplete Block Design
Other parameters::
sage: all(BIBD_from_arc_in_desarguesian_projective_plane(n,k,existence=True)
....: for n,k in
....: [(120, 8), (232, 8), (456, 8), (904, 8), (496, 16),
....: (976, 16), (1936, 16), (2016, 32), (4000, 32), (8128, 64)])
True
Of course, not all can be built this way::
sage: BIBD_from_arc_in_desarguesian_projective_plane(7,3,existence=True)
False
sage: BIBD_from_arc_in_desarguesian_projective_plane(7,3)
Traceback (most recent call last):
...
ValueError: This function cannot produce a (7,3,1)-BIBD
REFERENCE:
.. [Denniston69] R. H. F. Denniston,
Some maximal arcs in finite projective planes.
Journal of Combinatorial Theory 6, no. 3 (1969): 317-319.
http://dx.doi.org/10.1016/S0021-9800(69)80095-5
"""
q = (n-1)//(k-1)-1
if (k % 2 or
q % 2 or
q <= k or
n != (k-1)*(q+1)+1 or
not is_prime_power(k) or
not is_prime_power(q)):
if existence:
return False
raise ValueError("This function cannot produce a ({},{},1)-BIBD".format(n,k))
if existence:
return True
n = k
# From now on, the code assumes the notations of [Denniston69] for n,q, so
# that the BIBD returned by the method will have the requested parameters.
from sage.rings.finite_rings.constructor import FiniteField as GF
from sage.libs.gap.libgap import libgap
from sage.matrix.constructor import Matrix
K = GF(q,'a')
one = K.one()
# An irreducible quadratic form over K[X,Y]
GO = libgap.GeneralOrthogonalGroup(-1,2,q)
M = libgap.InvariantQuadraticForm(GO)['matrix']
M = Matrix(M)
M = M.change_ring(K)
Q = lambda xx,yy : M[0,0]*xx**2+(M[0,1]+M[1,0])*xx*yy+M[1,1]*yy**2
# Here, the additive subgroup H (of order n) of K mentioned in
# [Denniston69] is the set of all elements of K of degree < log_n
# (seeing elements of K as polynomials in 'a')
K_iter = list(K) # faster iterations
log_n = is_prime_power(n,get_data=True)[1]
C = [(x,y,one)
for x in K_iter
for y in K_iter
if Q(x,y).polynomial().degree() < log_n]
from sage.combinat.designs.block_design import DesarguesianProjectivePlaneDesign
return DesarguesianProjectivePlaneDesign(q).trace(C)._blocks
def hadamard_matrix_paleyII(n):
"""
Implements the Paley type II construction.
The Paley type II case corresponds to the case `p \cong 1 \mod{4}` for a
prime `p` (see [Hora]_).
EXAMPLES::
sage: sage.combinat.matrices.hadamard_matrix.hadamard_matrix_paleyII(12).det()
2985984
sage: 12^6
2985984
We note that the method returns a normalised Hadamard matrix ::
sage: sage.combinat.matrices.hadamard_matrix.hadamard_matrix_paleyII(12)
[ 1 1| 1 1| 1 1| 1 1| 1 1| 1 1]
[ 1 -1|-1 1|-1 1|-1 1|-1 1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 -1| 1 1|-1 -1|-1 -1| 1 1]
[ 1 1|-1 -1| 1 -1|-1 1|-1 1| 1 -1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 1| 1 -1| 1 1|-1 -1|-1 -1]
[ 1 1| 1 -1|-1 -1| 1 -1|-1 1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1|-1 -1| 1 1| 1 -1| 1 1|-1 -1]
[ 1 1|-1 1| 1 -1|-1 -1| 1 -1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1|-1 -1|-1 -1| 1 1| 1 -1| 1 1]
[ 1 1|-1 1|-1 1| 1 -1|-1 -1| 1 -1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 1|-1 -1|-1 -1| 1 1| 1 -1]
[ 1 1| 1 -1|-1 1|-1 1| 1 -1|-1 -1]
TESTS::
sage: from sage.combinat.matrices.hadamard_matrix import (hadamard_matrix_paleyII, is_hadamard_matrix)
sage: test_cases = [2*(x+1) for x in range(50) if is_prime_power(x) and x%4==1]
sage: all(is_hadamard_matrix(hadamard_matrix_paleyII(n),normalized=True,verbose=True)
....: for n in test_cases)
True
"""
q = n // 2 - 1
if not (n % 2 == 0 and is_prime_power(q) and (q % 4 == 1)):
raise ValueError(
"The order %s is not covered by the Paley type II construction." %
n)
from sage.rings.finite_rings.finite_field_constructor import FiniteField
K = FiniteField(q, 'x')
K_list = list(K)
K_list.insert(0, K.zero())
H = matrix(ZZ, [[(1 if (x - y).is_square() else -1) for x in K_list]
for y in K_list])
for i in range(q + 1):
H[0, i] = 1
H[i, 0] = 1
H[i, i] = 0
tr = {
0: matrix(2, 2, [1, -1, -1, -1]),
1: matrix(2, 2, [1, 1, 1, -1]),
-1: matrix(2, 2, [-1, -1, -1, 1])
}
H = block_matrix(q + 1, q + 1, [tr[v] for r in H for v in r])
return normalise_hadamard(H)
def v_5_1_BIBD(v, check=True):
r"""
Return a `(v,5,1)`-BIBD.
This method follows the constuction from [ClaytonSmith]_.
INPUT:
- ``v`` (integer)
.. SEEALSO::
* :func:`balanced_incomplete_block_design`
EXAMPLES::
sage: from sage.combinat.designs.bibd import v_5_1_BIBD
sage: i = 0
sage: while i<200:
....: i += 20
....: _ = v_5_1_BIBD(i+1)
....: _ = v_5_1_BIBD(i+5)
TESTS:
Check that the needed difference families are there::
sage: for v in [21,41,61,81,141,161,281]:
....: assert designs.difference_family(v,5,existence=True)
....: _ = designs.difference_family(v,5)
"""
v = int(v)
assert (v > 1)
assert (v%20 == 5 or v%20 == 1) # note: equivalent to (v-1)%4 == 0 and (v*(v-1))%20 == 0
# Lemma 27
if v%5 == 0 and (v//5)%4 == 1 and is_prime_power(v//5):
bibd = BIBD_5q_5_for_q_prime_power(v//5)
# Lemma 28
elif v in [21,41,61,81,141,161,281]:
from difference_family import difference_family
G,D = difference_family(v,5)
bibd = BIBD_from_difference_family(G, D, check=False)
# Lemma 29
elif v == 165:
bibd = BIBD_from_PBD(v_5_1_BIBD(41,check=False),165,5,check=False)
elif v == 181:
bibd = BIBD_from_PBD(v_5_1_BIBD(45,check=False),181,5,check=False)
elif v in (201,285,301,401,421,425):
# Call directly the BIBD_from_TD function
# note: there are (201,5,1) and (421,5)-difference families that can be
# obtained from the general constructor
bibd = BIBD_from_TD(v,5)
# Theorem 31.2
elif (v-1)//4 in [80, 81, 85, 86, 90, 91, 95, 96, 110, 111, 115, 116, 120, 121, 250, 251, 255, 256, 260, 261, 265, 266, 270, 271]:
r = (v-1)//4
if r <= 96:
k,t,u = 5, 16, r-80
elif r <= 121:
k,t,u = 10, 11, r-110
else:
k,t,u = 10, 25, r-250
bibd = BIBD_from_PBD(PBD_from_TD(k,t,u),v,5,check=False)
else:
r,s,t,u = _get_r_s_t_u(v)
bibd = BIBD_from_PBD(PBD_from_TD(5,t,u),v,5,check=False)
if check:
assert is_pairwise_balanced_design(bibd,v,[5])
return bibd
def hadamard_matrix(n, existence=False, check=True):
r"""
Tries to construct a Hadamard matrix using a combination of Paley
and Sylvester constructions.
INPUT:
- ``n`` (integer) -- dimension of the matrix
- ``existence`` (boolean) -- whether to build the matrix or merely query if
a construction is available in Sage. When set to ``True``, the function
returns:
- ``True`` -- meaning that Sage knows how to build the matrix
- ``Unknown`` -- meaning that Sage does not know how to build the
matrix, although the matrix may exist (see :mod:`sage.misc.unknown`).
- ``False`` -- meaning that the matrix does not exist.
- ``check`` (boolean) -- whether to check that output is correct before
returning it. As this is expected to be useless (but we are cautious
guys), you may want to disable it whenever you want speed. Set to ``True``
by default.
EXAMPLES::
sage: hadamard_matrix(12).det()
2985984
sage: 12^6
2985984
sage: hadamard_matrix(1)
[1]
sage: hadamard_matrix(2)
[ 1 1]
[ 1 -1]
sage: hadamard_matrix(8) # random
[ 1 1 1 1 1 1 1 1]
[ 1 -1 1 -1 1 -1 1 -1]
[ 1 1 -1 -1 1 1 -1 -1]
[ 1 -1 -1 1 1 -1 -1 1]
[ 1 1 1 1 -1 -1 -1 -1]
[ 1 -1 1 -1 -1 1 -1 1]
[ 1 1 -1 -1 -1 -1 1 1]
[ 1 -1 -1 1 -1 1 1 -1]
sage: hadamard_matrix(8).det() == 8^4
True
We note that :func:`hadamard_matrix` returns a normalised Hadamard matrix
(the entries in the first row and column are all +1) ::
sage: hadamard_matrix(12) # random
[ 1 1| 1 1| 1 1| 1 1| 1 1| 1 1]
[ 1 -1|-1 1|-1 1|-1 1|-1 1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 -1| 1 1|-1 -1|-1 -1| 1 1]
[ 1 1|-1 -1| 1 -1|-1 1|-1 1| 1 -1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 1| 1 -1| 1 1|-1 -1|-1 -1]
[ 1 1| 1 -1|-1 -1| 1 -1|-1 1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1|-1 -1| 1 1| 1 -1| 1 1|-1 -1]
[ 1 1|-1 1| 1 -1|-1 -1| 1 -1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1|-1 -1|-1 -1| 1 1| 1 -1| 1 1]
[ 1 1|-1 1|-1 1| 1 -1|-1 -1| 1 -1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 1|-1 -1|-1 -1| 1 1| 1 -1]
[ 1 1| 1 -1|-1 1|-1 1| 1 -1|-1 -1]
TESTS::
sage: matrix.hadamard(10,existence=True)
False
sage: matrix.hadamard(12,existence=True)
True
sage: matrix.hadamard(92,existence=True)
Unknown
sage: matrix.hadamard(10)
Traceback (most recent call last):
...
ValueError: The Hadamard matrix of order 10 does not exist
"""
if not (n % 4 == 0) and (n > 2):
if existence:
return False
raise ValueError("The Hadamard matrix of order %s does not exist" % n)
if n == 2:
if existence:
return True
M = matrix([[1, 1], [1, -1]])
elif n == 1:
if existence:
return True
M = matrix([1])
elif is_prime_power(n // 2 - 1) and (n // 2 - 1) % 4 == 1:
if existence:
return True
M = hadamard_matrix_paleyII(n)
elif n == 4 or n % 8 == 0:
if existence:
return hadamard_matrix(n // 2, existence=True)
had = hadamard_matrix(n // 2, check=False)
chad1 = matrix([list(r) + list(r) for r in had.rows()])
mhad = (-1) * had
R = len(had.rows())
chad2 = matrix(
[list(had.rows()[i]) + list(mhad.rows()[i]) for i in range(R)])
M = chad1.stack(chad2)
elif is_prime_power(n - 1) and (n - 1) % 4 == 3:
if existence:
return True
M = hadamard_matrix_paleyI(n)
else:
if existence:
return Unknown
raise ValueError(
"The Hadamard matrix of order %s is not yet implemented." % n)
if check:
assert is_hadamard_matrix(M, normalized=True)
return M
def hadamard_matrix(n,existence=False, check=True):
r"""
Tries to construct a Hadamard matrix using a combination of Paley
and Sylvester constructions.
INPUT:
- ``n`` (integer) -- dimension of the matrix
- ``existence`` (boolean) -- whether to build the matrix or merely query if
a construction is available in Sage. When set to ``True``, the function
returns:
- ``True`` -- meaning that Sage knows how to build the matrix
- ``Unknown`` -- meaning that Sage does not know how to build the
matrix, although the matrix may exist (see :mod:`sage.misc.unknown`).
- ``False`` -- meaning that the matrix does not exist.
- ``check`` (boolean) -- whether to check that output is correct before
returning it. As this is expected to be useless (but we are cautious
guys), you may want to disable it whenever you want speed. Set to ``True``
by default.
EXAMPLES::
sage: hadamard_matrix(12).det()
2985984
sage: 12^6
2985984
sage: hadamard_matrix(1)
[1]
sage: hadamard_matrix(2)
[ 1 1]
[ 1 -1]
sage: hadamard_matrix(8) # random
[ 1 1 1 1 1 1 1 1]
[ 1 -1 1 -1 1 -1 1 -1]
[ 1 1 -1 -1 1 1 -1 -1]
[ 1 -1 -1 1 1 -1 -1 1]
[ 1 1 1 1 -1 -1 -1 -1]
[ 1 -1 1 -1 -1 1 -1 1]
[ 1 1 -1 -1 -1 -1 1 1]
[ 1 -1 -1 1 -1 1 1 -1]
sage: hadamard_matrix(8).det() == 8^4
True
We note that the method `hadamard_matrix()` returns a normalised Hadamard matrix
(the entries in the first row and column are all +1) ::
sage: hadamard_matrix(12) # random
[ 1 1| 1 1| 1 1| 1 1| 1 1| 1 1]
[ 1 -1|-1 1|-1 1|-1 1|-1 1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 -1| 1 1|-1 -1|-1 -1| 1 1]
[ 1 1|-1 -1| 1 -1|-1 1|-1 1| 1 -1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 1| 1 -1| 1 1|-1 -1|-1 -1]
[ 1 1| 1 -1|-1 -1| 1 -1|-1 1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1|-1 -1| 1 1| 1 -1| 1 1|-1 -1]
[ 1 1|-1 1| 1 -1|-1 -1| 1 -1|-1 1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1|-1 -1|-1 -1| 1 1| 1 -1| 1 1]
[ 1 1|-1 1|-1 1| 1 -1|-1 -1| 1 -1]
[-----+-----+-----+-----+-----+-----]
[ 1 -1| 1 1|-1 -1|-1 -1| 1 1| 1 -1]
[ 1 1| 1 -1|-1 1|-1 1| 1 -1|-1 -1]
TESTS::
sage: matrix.hadamard(10,existence=True)
False
sage: matrix.hadamard(12,existence=True)
True
sage: matrix.hadamard(92,existence=True)
Unknown
sage: matrix.hadamard(10)
Traceback (most recent call last):
...
ValueError: The Hadamard matrix of order 10 does not exist
"""
if not(n % 4 == 0) and (n > 2):
if existence:
return False
raise ValueError("The Hadamard matrix of order %s does not exist" % n)
if n == 2:
if existence:
return True
M = matrix([[1, 1], [1, -1]])
elif n == 1:
if existence:
return True
M = matrix([1])
elif is_prime_power(n//2 - 1) and (n//2 - 1) % 4 == 1:
if existence:
return True
M = hadamard_matrix_paleyII(n)
elif n == 4 or n % 8 == 0:
if existence:
return hadamard_matrix(n//2,existence=True)
had = hadamard_matrix(n//2,check=False)
chad1 = matrix([list(r) + list(r) for r in had.rows()])
mhad = (-1) * had
R = len(had.rows())
chad2 = matrix([list(had.rows()[i]) + list(mhad.rows()[i])
for i in range(R)])
M = chad1.stack(chad2)
elif is_prime_power(n - 1) and (n - 1) % 4 == 3:
if existence:
return True
M = hadamard_matrix_paleyI(n)
else:
if existence:
return Unknown
raise ValueError("The Hadamard matrix of order %s is not yet implemented." % n)
if check:
assert is_hadamard_matrix(M, normalized=True)
return M
def regular_symmetric_hadamard_matrix_with_constant_diagonal(
n, e, existence=False):
r"""
Return a Regular Symmetric Hadamard Matrix with Constant Diagonal.
A Hadamard matrix is said to be *regular* if its rows all sum to the same
value.
For `\epsilon\in\{-1,+1\}`, we say that `M` is a `(n,\epsilon)-RSHCD` if
`M` is a regular symmetric Hadamard matrix with constant diagonal
`\delta\in\{-1,+1\}` and row sums all equal to `\delta \epsilon
\sqrt(n)`. For more information, see [HX10]_ or 10.5.1 in
[BH12]_. For the case `n=324`, see :func:`RSHCD_324` and [CP16]_.
INPUT:
- ``n`` (integer) -- side of the matrix
- ``e`` -- one of `-1` or `+1`, equal to the value of `\epsilon`
EXAMPLES::
sage: from sage.combinat.matrices.hadamard_matrix import regular_symmetric_hadamard_matrix_with_constant_diagonal
sage: regular_symmetric_hadamard_matrix_with_constant_diagonal(4,1)
[ 1 1 1 -1]
[ 1 1 -1 1]
[ 1 -1 1 1]
[-1 1 1 1]
sage: regular_symmetric_hadamard_matrix_with_constant_diagonal(4,-1)
[ 1 -1 -1 -1]
[-1 1 -1 -1]
[-1 -1 1 -1]
[-1 -1 -1 1]
Other hardcoded values::
sage: for n,e in [(36,1),(36,-1),(100,1),(100,-1),(196, 1)]: # long time
....: print(repr(regular_symmetric_hadamard_matrix_with_constant_diagonal(n,e)))
36 x 36 dense matrix over Integer Ring
36 x 36 dense matrix over Integer Ring
100 x 100 dense matrix over Integer Ring
100 x 100 dense matrix over Integer Ring
196 x 196 dense matrix over Integer Ring
sage: for n,e in [(324,1),(324,-1)]: # not tested - long time, tested in RSHCD_324
....: print(repr(regular_symmetric_hadamard_matrix_with_constant_diagonal(n,e)))
324 x 324 dense matrix over Integer Ring
324 x 324 dense matrix over Integer Ring
From two close prime powers::
sage: regular_symmetric_hadamard_matrix_with_constant_diagonal(64,-1)
64 x 64 dense matrix over Integer Ring (use the '.str()' method to see the entries)
From a prime power and a conference matrix::
sage: regular_symmetric_hadamard_matrix_with_constant_diagonal(676,1) # long time
676 x 676 dense matrix over Integer Ring (use the '.str()' method to see the entries)
Recursive construction::
sage: regular_symmetric_hadamard_matrix_with_constant_diagonal(144,-1)
144 x 144 dense matrix over Integer Ring (use the '.str()' method to see the entries)
REFERENCE:
.. [BH12] \A. Brouwer and W. Haemers,
Spectra of graphs,
Springer, 2012,
http://homepages.cwi.nl/~aeb/math/ipm/ipm.pdf
.. [HX10] \W. Haemers and Q. Xiang,
Strongly regular graphs with parameters `(4m^4,2m^4+m^2,m^4+m^2,m^4+m^2)` exist for all `m>1`,
European Journal of Combinatorics,
Volume 31, Issue 6, August 2010, Pages 1553-1559,
:doi:`10.1016/j.ejc.2009.07.009`
"""
if existence and (n, e) in _rshcd_cache:
return _rshcd_cache[n, e]
from sage.graphs.strongly_regular_db import strongly_regular_graph
def true():
_rshcd_cache[n, e] = True
return True
M = None
if abs(e) != 1:
raise ValueError
sqn = None
if is_square(n):
sqn = int(sqrt(n))
if n < 0:
if existence:
return False
raise ValueError
elif n == 4:
if existence:
return true()
if e == 1:
M = J(4) - 2 * matrix(4, [[int(i + j == 3) for i in range(4)]
for j in range(4)])
else:
M = -J(4) + 2 * I(4)
elif n == 36:
if existence:
return true()
if e == 1:
M = strongly_regular_graph(36, 15, 6, 6).adjacency_matrix()
M = J(36) - 2 * M
else:
M = strongly_regular_graph(36, 14, 4, 6).adjacency_matrix()
M = -J(36) + 2 * M + 2 * I(36)
elif n == 100:
if existence:
return true()
if e == -1:
M = strongly_regular_graph(100, 44, 18, 20).adjacency_matrix()
M = 2 * M - J(100) + 2 * I(100)
else:
M = strongly_regular_graph(100, 45, 20, 20).adjacency_matrix()
M = J(100) - 2 * M
elif n == 196 and e == 1:
if existence:
return true()
M = strongly_regular_graph(196, 91, 42, 42).adjacency_matrix()
M = J(196) - 2 * M
elif n == 324:
if existence:
return true()
M = RSHCD_324(e)
elif (e == 1 and n % 16 == 0 and not sqn is None
and is_prime_power(sqn - 1) and is_prime_power(sqn + 1)):
if existence:
return true()
M = -rshcd_from_close_prime_powers(sqn)
elif (e == 1 and not sqn is None and sqn % 4 == 2
and True == strongly_regular_graph(sqn - 1, (sqn - 2) // 2,
(sqn - 6) // 4,
existence=True)
and is_prime_power(ZZ(sqn + 1))):
if existence:
return true()
M = rshcd_from_prime_power_and_conference_matrix(sqn + 1)
# Recursive construction: the kronecker product of two RSHCD is a RSHCD
else:
from itertools import product
for n1, e1 in product(divisors(n)[1:-1], [-1, 1]):
e2 = e1 * e
n2 = n // n1
if (regular_symmetric_hadamard_matrix_with_constant_diagonal(
n1, e1, existence=True) and
regular_symmetric_hadamard_matrix_with_constant_diagonal(
n2, e2, existence=True)):
if existence:
return true()
M1 = regular_symmetric_hadamard_matrix_with_constant_diagonal(
n1, e1)
M2 = regular_symmetric_hadamard_matrix_with_constant_diagonal(
n2, e2)
M = M1.tensor_product(M2)
break
if M is None:
from sage.misc.unknown import Unknown
_rshcd_cache[n, e] = Unknown
if existence:
return Unknown
raise ValueError("I do not know how to build a {}-RSHCD".format(
(n, e)))
assert M * M.transpose() == n * I(n)
assert set(map(sum, M)) == {ZZ(e * sqn)}
return M
def hadamard_matrix_paleyI(n, normalize=True):
"""
Implements the Paley type I construction.
The Paley type I case corresponds to the case `p \cong 3 \mod{4}` for a
prime `p` (see [Hora]_).
INPUT:
- ``n`` -- the matrix size
- ``normalize`` (boolean) -- whether to normalize the result.
EXAMPLES:
We note that this method by default returns a normalised Hadamard matrix ::
sage: from sage.combinat.matrices.hadamard_matrix import hadamard_matrix_paleyI
sage: hadamard_matrix_paleyI(4)
[ 1 1 1 1]
[ 1 -1 1 -1]
[ 1 -1 -1 1]
[ 1 1 -1 -1]
Otherwise, it returns a skew Hadamard matrix `H`, i.e. `H=S+I`, with
`S=-S^\top` ::
sage: M=hadamard_matrix_paleyI(4, normalize=False); M
[ 1 1 1 1]
[-1 1 1 -1]
[-1 -1 1 1]
[-1 1 -1 1]
sage: S=M-identity_matrix(4); -S==S.T
True
TESTS::
sage: from sage.combinat.matrices.hadamard_matrix import is_hadamard_matrix
sage: test_cases = [x+1 for x in range(100) if is_prime_power(x) and x%4==3]
sage: all(is_hadamard_matrix(hadamard_matrix_paleyI(n),normalized=True,verbose=True)
....: for n in test_cases)
True
sage: all(is_hadamard_matrix(hadamard_matrix_paleyI(n,normalize=False),verbose=True)
....: for n in test_cases)
True
"""
p = n - 1
if not(is_prime_power(p) and (p % 4 == 3)):
raise ValueError("The order %s is not covered by the Paley type I construction." % n)
from sage.rings.finite_rings.finite_field_constructor import FiniteField
K = FiniteField(p,'x')
K_list = list(K)
K_list.insert(0,K.zero())
H = matrix(ZZ, [[(1 if (x-y).is_square() else -1)
for x in K_list]
for y in K_list])
for i in range(n):
H[i,0] = -1
H[0,i] = 1
if normalize:
for i in range(n):
H[i,i] = -1
H = normalise_hadamard(H)
return H
def hadamard_matrix_paleyI(n, normalize=True):
"""
Implements the Paley type I construction.
The Paley type I case corresponds to the case `p \cong 3 \mod{4}` for a
prime `p` (see [Hora]_).
INPUT:
- ``n`` -- the matrix size
- ``normalize`` (boolean) -- whether to normalize the result.
EXAMPLES:
We note that this method by default returns a normalised Hadamard matrix ::
sage: from sage.combinat.matrices.hadamard_matrix import hadamard_matrix_paleyI
sage: hadamard_matrix_paleyI(4)
[ 1 1 1 1]
[ 1 -1 1 -1]
[ 1 -1 -1 1]
[ 1 1 -1 -1]
Otherwise, it returns a skew Hadamard matrix `H`, i.e. `H=S+I`, with
`S=-S^\top` ::
sage: M=hadamard_matrix_paleyI(4, normalize=False); M
[ 1 1 1 1]
[-1 1 1 -1]
[-1 -1 1 1]
[-1 1 -1 1]
sage: S=M-identity_matrix(4); -S==S.T
True
TESTS::
sage: from sage.combinat.matrices.hadamard_matrix import is_hadamard_matrix
sage: test_cases = [x+1 for x in range(100) if is_prime_power(x) and x%4==3]
sage: all(is_hadamard_matrix(hadamard_matrix_paleyI(n),normalized=True,verbose=True)
....: for n in test_cases)
True
sage: all(is_hadamard_matrix(hadamard_matrix_paleyI(n,normalize=False),verbose=True)
....: for n in test_cases)
True
"""
p = n - 1
if not (is_prime_power(p) and (p % 4 == 3)):
raise ValueError(
"The order %s is not covered by the Paley type I construction." %
n)
from sage.rings.finite_rings.finite_field_constructor import FiniteField
K = FiniteField(p, 'x')
K_list = list(K)
K_list.insert(0, K.zero())
H = matrix(ZZ, [[(1 if (x - y).is_square() else -1) for x in K_list]
for y in K_list])
for i in range(n):
H[i, 0] = -1
H[0, i] = 1
if normalize:
for i in range(n):
H[i, i] = -1
H = normalise_hadamard(H)
return H
def projective_plane(n, check=True, existence=False):
r"""
Return a projective plane of order ``n`` as a 2-design.
A finite projective plane is a 2-design with `n^2+n+1` lines (or blocks) and
`n^2+n+1` points. For more information on finite projective planes, see the
:wikipedia:`Projective_plane#Finite_projective_planes`.
If no construction is possible, then the function raises a ``EmptySetError``
whereas if no construction is available the function raises a
``NotImplementedError``.
INPUT:
- ``n`` -- the finite projective plane's order
EXAMPLES::
sage: designs.projective_plane(2)
(7,3,1)-Balanced Incomplete Block Design
sage: designs.projective_plane(3)
(13,4,1)-Balanced Incomplete Block Design
sage: designs.projective_plane(4)
(21,5,1)-Balanced Incomplete Block Design
sage: designs.projective_plane(5)
(31,6,1)-Balanced Incomplete Block Design
sage: designs.projective_plane(6)
Traceback (most recent call last):
...
EmptySetError: By the Bruck-Ryser theorem, no projective plane of order 6 exists.
sage: designs.projective_plane(10)
Traceback (most recent call last):
...
EmptySetError: No projective plane of order 10 exists by C. Lam, L. Thiel and S. Swiercz "The nonexistence of finite projective planes of order 10" (1989), Canad. J. Math.
sage: designs.projective_plane(12)
Traceback (most recent call last):
...
NotImplementedError: If such a projective plane exists, we do not know how to build it.
sage: designs.projective_plane(14)
Traceback (most recent call last):
...
EmptySetError: By the Bruck-Ryser theorem, no projective plane of order 14 exists.
TESTS::
sage: designs.projective_plane(2197, existence=True)
True
sage: designs.projective_plane(6, existence=True)
False
sage: designs.projective_plane(10, existence=True)
False
sage: designs.projective_plane(12, existence=True)
Unknown
"""
from sage.combinat.designs.bibd import BruckRyserChowla_check
if n <= 1:
if existence:
return False
raise EmptySetError("There is no projective plane of order <= 1")
if n == 10:
if existence:
return False
ref = ("C. Lam, L. Thiel and S. Swiercz \"The nonexistence of finite "
"projective planes of order 10\" (1989), Canad. J. Math.")
raise EmptySetError("No projective plane of order 10 exists by %s" %
ref)
if BruckRyserChowla_check(n * n + n + 1, n + 1, 1) is False:
if existence:
return False
raise EmptySetError("By the Bruck-Ryser theorem, no projective"
" plane of order {} exists.".format(n))
if not is_prime_power(n):
if existence:
return Unknown
raise NotImplementedError("If such a projective plane exists, we do "
"not know how to build it.")
if existence:
return True
else:
return DesarguesianProjectivePlaneDesign(n,
point_coordinates=False,
check=check)
def kirkman_triple_system(v,existence=False):
r"""
Return a Kirkman Triple System on `v` points.
A Kirkman Triple System `KTS(v)` is a resolvable Steiner Triple System. It
exists if and only if `v\equiv 3\pmod{6}`.
INPUT:
- `n` (integer)
- ``existence`` (boolean; ``False`` by default) -- whether to build the
`KTS(n)` or only answer whether it exists.
.. SEEALSO::
:meth:`IncidenceStructure.is_resolvable`
EXAMPLES:
A solution to Kirkmman's original problem::
sage: kts = designs.kirkman_triple_system(15)
sage: classes = kts.is_resolvable(1)[1]
sage: names = '0123456789abcde'
sage: to_name = lambda (r,s,t): ' '+names[r]+names[s]+names[t]+' '
sage: rows = [' '.join(('Day {}'.format(i) for i in range(1,8)))]
sage: rows.extend(' '.join(map(to_name,row)) for row in zip(*classes))
sage: print '\n'.join(rows)
Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7
07e 18e 29e 3ae 4be 5ce 6de
139 24a 35b 46c 05d 167 028
26b 03c 14d 257 368 049 15a
458 569 06a 01b 12c 23d 347
acd 7bd 78c 89d 79a 8ab 9bc
TESTS::
sage: for i in range(3,300,6):
....: _ = designs.kirkman_triple_system(i)
"""
if v%6 != 3:
if existence:
return False
raise ValueError("There is no KTS({}) as v!=3 mod(6)".format(v))
if existence:
return False
elif v == 3:
return BalancedIncompleteBlockDesign(3,[[0,1,2]],k=3,lambd=1)
elif v == 9:
classes = [[[0, 1, 5], [2, 6, 7], [3, 4, 8]],
[[1, 6, 8], [3, 5, 7], [0, 2, 4]],
[[1, 4, 7], [0, 3, 6], [2, 5, 8]],
[[4, 5, 6], [0, 7, 8], [1, 2, 3]]]
KTS = BalancedIncompleteBlockDesign(v,[tr for cl in classes for tr in cl],k=3,lambd=1,copy=False)
KTS._classes = classes
return KTS
# Construction 1.1 from [Stinson91] (originally Theorem 6 from [RCW71])
#
# For all prime powers q=1 mod 6, there exists a KTS(2q+1)
elif ((v-1)//2)%6 == 1 and is_prime_power((v-1)//2):
from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
q = (v-1)//2
K = GF(q,'x')
a = K.primitive_element()
t = (q-1)/6
# m is the solution of a^m=(a^t+1)/2
from sage.groups.generic import discrete_log
m = discrete_log((a**t+1)/2, a)
assert 2*a**m == a**t+1
# First parallel class
first_class = [[(0,1),(0,2),'inf']]
b0 = K.one(); b1 = a**t; b2 = a**m
first_class.extend([(b0*a**i,1),(b1*a**i,1),(b2*a**i,2)]
for i in range(t)+range(2*t,3*t)+range(4*t,5*t))
b0 = a**(m+t); b1=a**(m+3*t); b2=a**(m+5*t)
first_class.extend([[(b0*a**i,2),(b1*a**i,2),(b2*a**i,2)]
for i in range(t)])
# Action of K on the points
action = lambda v,x : (v+x[0],x[1]) if len(x) == 2 else x
# relabel to integer
relabel = {(p,x): i+(x-1)*q
for i,p in enumerate(K)
for x in [1,2]}
relabel['inf'] = 2*q
classes = [[[relabel[action(p,x)] for x in tr] for tr in first_class]
for p in K]
KTS = BalancedIncompleteBlockDesign(v,[tr for cl in classes for tr in cl],k=3,lambd=1,copy=False)
KTS._classes = classes
return KTS
# Construction 1.2 from [Stinson91] (originally Theorem 5 from [RCW71])
#
# For all prime powers q=1 mod 6, there exists a KTS(3q)
elif (v//3)%6 == 1 and is_prime_power(v//3):
from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
q = v//3
K = GF(q,'x')
a = K.primitive_element()
t = (q-1)/6
A0 = [(0,0),(0,1),(0,2)]
B = [[(a**i,j),(a**(i+2*t),j),(a**(i+4*t),j)] for j in range(3)
for i in range(t)]
A = [[(a**i,0),(a**(i+2*t),1),(a**(i+4*t),2)] for i in range(6*t)]
# Action of K on the points
action = lambda v,x: (v+x[0],x[1])
# relabel to integer
relabel = {(p,j): i+j*q
for i,p in enumerate(K)
for j in range(3)}
B0 = [A0] + B + A[t:2*t] + A[3*t:4*t] + A[5*t:6*t]
# Classes
classes = [[[relabel[action(p,x)] for x in tr] for tr in B0]
for p in K]
for i in range(t)+range(2*t,3*t)+range(4*t,5*t):
classes.append([[relabel[action(p,x)] for x in A[i]] for p in K])
KTS = BalancedIncompleteBlockDesign(v,[tr for cl in classes for tr in cl],k=3,lambd=1,copy=False)
KTS._classes = classes
return KTS
else:
# This is Lemma IX.6.4 from [BJL99].
#
# This construction takes a (v,{4,7})-PBD. All points are doubled (x has
# a copy x'), and an infinite point \infty is added.
#
# On all blocks of 2*4 points we "paste" a KTS(2*4+1) using the infinite
# point, in such a way that all {x,x',infty} are set of the design. We
# do the same for blocks with 2*7 points using a KTS(2*7+1).
#
# Note that the triples of points equal to {x,x',\infty} will be added
# several times.
#
# As all those subdesigns are resolvable, each class of the KTS(n) is
# obtained by considering a set {x,x',\infty} and all sets of all
# parallel classes of the subdesign which contain this set.
# We create the small KTS(n') we need, and relabel them such that
# 01(n'-1),23(n'-1),... are blocks of the design.
gdd4 = kirkman_triple_system(9)
gdd7 = kirkman_triple_system(15)
X = [B for B in gdd4 if 8 in B]
for b in X:
b.remove(8)
X = sum(X, []) + [8]
gdd4.relabel({v:i for i,v in enumerate(X)})
gdd4 = gdd4.is_resolvable(True)[1] # the relabeled classes
X = [B for B in gdd7 if 14 in B]
for b in X:
b.remove(14)
X = sum(X, []) + [14]
gdd7.relabel({v:i for i,v in enumerate(X)})
gdd7 = gdd7.is_resolvable(True)[1] # the relabeled classes
# The first parallel class contains 01(n'-1), the second contains
# 23(n'-1), etc..
# Then remove the blocks containing (n'-1)
for B in gdd4:
for i,b in enumerate(B):
if 8 in b: j = min(b); del B[i]; B.insert(0,j); break
gdd4.sort()
for B in gdd4:
B.pop(0)
for B in gdd7:
for i,b in enumerate(B):
if 14 in b: j = min(b); del B[i]; B.insert(0,j); break
gdd7.sort()
for B in gdd7:
B.pop(0)
# Pasting the KTS(n') without {x,x',\infty} blocks
classes = [[] for i in range((v-1)/2)]
gdd = {4:gdd4, 7: gdd7}
for B in PBD_4_7((v-1)//2,check=False):
for i,classs in enumerate(gdd[len(B)]):
classes[B[i]].extend([[2*B[x//2]+x%2 for x in BB] for BB in classs])
# The {x,x',\infty} blocks
for i,classs in enumerate(classes):
classs.append([2*i,2*i+1,v-1])
KTS = BalancedIncompleteBlockDesign(v,
blocks = [tr for cl in classes for tr in cl],
k=3,
lambd=1,
check=True,
copy =False)
KTS._classes = classes
assert KTS.is_resolvable()
return KTS
def CossidentePenttilaGraph(q):
r"""
Cossidente-Penttila `((q^3+1)(q+1)/2,(q^2+1)(q-1)/2,(q-3)/2,(q-1)^2/2)`-strongly regular graph
For each odd prime power `q`, one can partition the points of the `O_6^-(q)`-generalized
quadrange `GQ(q,q^2)` into two parts, so that on any of them the induced subgraph of
the point graph of the GQ has parameters as above [CP05]_.
Directly follwing the construction in [CP05]_ is not efficient,
as one then needs to construct the dual `GQ(q^2,q)`. Thus we
describe here a more efficient approach that we came up with, following a suggestion by
T.Penttila. Namely, this partition is invariant
under the subgroup `H=\Omega_3(q^2)<O_6^-(q)`. We build the appropriate `H`, which
leaves the form `B(X,Y,Z)=XY+Z^2` invariant, and
pick up two orbits of `H` on the `F_q`-points. One them is `B`-isotropic, and we
take the representative `(1:0:0)`. The other one corresponds to the points of
`PG(2,q^2)` that have all the lines on them either missing the conic specified by `B`, or
intersecting the conic in two points. We take `(1:1:e)` as the representative. It suffices
to pick `e` so that `e^2+1` is not a square in `F_{q^2}`. Indeed,
The conic can be viewed as the union of `\{(0:1:0)\}` and `\{(1:-t^2:t) | t \in F_{q^2}\}`.
The coefficients of a generic line on `(1:1:e)` are `[1:-1-eb:b]`, for `-1\neq eb`.
Thus, to make sure the intersection with the conic is always even, we need that the
discriminant of `1+(1+eb)t^2+tb=0` never vanishes, and this is if and only if
`e^2+1` is not a square. Further, we need to adjust `B`, by multiplying it by appropriately
chosen `\nu`, so that `(1:1:e)` becomes isotropic under the relative trace norm
`\nu B(X,Y,Z)+(\nu B(X,Y,Z))^q`. The latter is used then to define the graph.
INPUT:
- ``q`` -- an odd prime power.
EXAMPLES:
For `q=3` one gets Sims-Gewirtz graph. ::
sage: G=graphs.CossidentePenttilaGraph(3) # optional - gap_packages (grape)
sage: G.is_strongly_regular(parameters=True) # optional - gap_packages (grape)
(56, 10, 0, 2)
For `q>3` one gets new graphs. ::
sage: G=graphs.CossidentePenttilaGraph(5) # optional - gap_packages (grape)
sage: G.is_strongly_regular(parameters=True) # optional - gap_packages (grape)
(378, 52, 1, 8)
TESTS::
sage: G=graphs.CossidentePenttilaGraph(7) # optional - gap_packages (grape) # long time
sage: G.is_strongly_regular(parameters=True) # optional - gap_packages (grape) # long time
(1376, 150, 2, 18)
sage: graphs.CossidentePenttilaGraph(2)
Traceback (most recent call last):
...
ValueError: q(=2) must be an odd prime power
REFERENCES:
.. [CP05] \A.Cossidente and T.Penttila
Hemisystems on the Hermitian surface
Journal of London Math. Soc. 72(2005), 731-741
"""
p, k = is_prime_power(q,get_data=True)
if k==0 or p==2:
raise ValueError('q(={}) must be an odd prime power'.format(q))
from sage.libs.gap.libgap import libgap
from sage.misc.package import is_package_installed, PackageNotFoundError
if not is_package_installed('gap_packages'):
raise PackageNotFoundError('gap_packages')
adj_list=libgap.function_factory("""function(q)
local z, e, so, G, nu, G1, G0, B, T, s, O1, O2, x;
LoadPackage("grape");
G0:=SO(3,q^2);
so:=GeneratorsOfGroup(G0);
G1:=Group(Comm(so[1],so[2]),Comm(so[1],so[3]),Comm(so[2],so[3]));
B:=InvariantBilinearForm(G0).matrix;
z:=Z(q^2); e:=z; sqo:=(q^2-1)/2;
if IsInt(sqo/Order(e^2+z^0)) then
e:=z^First([2..q^2-2], x-> not IsInt(sqo/Order(z^(2*x)+z^0)));
fi;
nu:=z^First([0..q^2-2], x->z^x*(e^2+z^0)+(z^x*(e^2+z^0))^q=0*z);
T:=function(x)
local r;
r:=nu*x*B*x;
return r+r^q;
end;
s:=Group([Z(q)*IdentityMat(3,GF(q))]);
O1:=Orbit(G1, Set(Orbit(s,z^0*[1,0,0])), OnSets);
O2:=Orbit(G1, Set(Orbit(s,z^0*[1,1,e])), OnSets);
G:=Graph(G1,Concatenation(O1,O2),OnSets,
function(x,y) return x<>y and 0*z=T(x[1]+y[1]); end);
return List([1..OrderGraph(G)],x->Adjacency(G,x));
end;""")
adj = adj_list(q) # for each vertex, we get the list of vertices it is adjacent to
G = Graph(((i,int(j-1))
for i,ni in enumerate(adj) for j in ni),
format='list_of_edges', multiedges=False)
G.name('CossidentePenttila('+str(q)+')')
return G
def v_4_1_rbibd(v,existence=False):
r"""
Return a `(v,4,1)`-RBIBD.
INPUT:
- `n` (integer)
- ``existence`` (boolean; ``False`` by default) -- whether to build the
design or only answer whether it exists.
.. SEEALSO::
- :meth:`IncidenceStructure.is_resolvable`
- :func:`resolvable_balanced_incomplete_block_design`
.. NOTE::
A resolvable `(v,4,1)`-BIBD exists whenever `1\equiv 4\pmod(12)`. This
function, however, only implements a construction of `(v,4,1)`-BIBD such
that `v=3q+1\equiv 1\pmod{3}` where `q` is a prime power (see VII.7.5.a
from [BJL99]_).
EXAMPLE::
sage: rBIBD = designs.resolvable_balanced_incomplete_block_design(28,4)
sage: rBIBD.is_resolvable()
True
sage: rBIBD.is_t_design(return_parameters=True)
(True, (2, 28, 4, 1))
TESTS::
sage: for q in prime_powers(2,30):
....: if (3*q+1)%12 == 4:
....: _ = designs.resolvable_balanced_incomplete_block_design(3*q+1,4) # indirect doctest
"""
# Volume 1, VII.7.5.a from [BJL99]_
if v%3 != 1 or not is_prime_power((v-1)//3):
if existence:
return Unknown
raise NotImplementedError("I don't know how to build a ({},{},1)-RBIBD!".format(v,4))
from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
q = (v-1)//3
nn = (q-1)//4
G = GF(q,'x')
w = G.primitive_element()
e = w**(nn)
assert e**2 == -1
first_class = [[(w**i,j),(-w**i,j),(e*w**i,j+1),(-e*w**i,j+1)]
for i in range(nn) for j in range(3)]
first_class.append([(0,0),(0,1),(0,2),'inf'])
label = {p:i for i,p in enumerate(G)}
classes = [[[v-1 if x=='inf' else (x[1]%3)*q+label[x[0]+g] for x in S]
for S in first_class]
for g in G]
BIBD = BalancedIncompleteBlockDesign(v,
blocks = sum(classes,[]),
k=4,
check=True,
copy=False)
BIBD._classes = classes
assert BIBD.is_resolvable()
return BIBD
def NonisotropicUnitaryPolarGraph(m, q):
r"""
Returns the Graph `NU(m,q)`.
Returns the graph on nonisotropic, with respect to a nondegenerate
Hermitean form, points of the `(m-1)`-dimensional projective space over `F_q`,
with points adjacent whenever they lie on a tangent (to the set of isotropic points)
line.
For more information, see Sect. 9.9 of [BH12]_ and series C14 in [Hu75]_.
INPUT:
- ``m,q`` (integers) -- `q` must be a prime power.
EXAMPLES::
sage: g=graphs.NonisotropicUnitaryPolarGraph(5,2); g
NU(5, 2): Graph on 176 vertices
sage: g.is_strongly_regular(parameters=True)
(176, 135, 102, 108)
TESTS::
sage: graphs.NonisotropicUnitaryPolarGraph(4,2).is_strongly_regular(parameters=True)
(40, 27, 18, 18)
sage: graphs.NonisotropicUnitaryPolarGraph(4,3).is_strongly_regular(parameters=True) # long time
(540, 224, 88, 96)
sage: graphs.NonisotropicUnitaryPolarGraph(6,6)
Traceback (most recent call last):
...
ValueError: q must be a prime power
REFERENCE:
.. [Hu75] \X. L. Hubaut.
Strongly regular graphs.
Disc. Math. 13(1975), pp 357--381.
http://dx.doi.org/10.1016/0012-365X(75)90057-6
"""
p, k = is_prime_power(q,get_data=True)
if k==0:
raise ValueError('q must be a prime power')
from sage.libs.gap.libgap import libgap
from itertools import combinations
F=libgap.GF(q**2) # F_{q^2}
W=libgap.FullRowSpace(F, m) # F_{q^2}^m
B=libgap.Elements(libgap.Basis(W)) # the standard basis of W
if m % 2 != 0:
point = B[(m-1)/2]
else:
if p==2:
point = B[m/2] + F.PrimitiveRoot()*B[(m-2)/2]
else:
point = B[(m-2)/2] + B[m/2]
g = libgap.GeneralUnitaryGroup(m,q)
V = libgap.Orbit(g,point,libgap.OnLines) # orbit on nonisotropic points
gp = libgap.Action(g,V,libgap.OnLines) # make a permutation group
s = libgap.Subspace(W,[point, point+B[0]]) # a tangent line on point
# and the points there
sp = [libgap.Elements(libgap.Basis(x))[0] for x in libgap.Elements(s.Subspaces(1))]
h = libgap.Set(map(lambda x: libgap.Position(V, x), libgap.Intersection(V,sp))) # indices
L = libgap.Orbit(gp, h, libgap.OnSets) # orbit on the tangent lines
G = Graph()
for x in L: # every pair of points in the subspace is adjacent to each other in G
G.add_edges(combinations(x, 2))
G.relabel()
G.name("NU" + str((m, q)))
return G
def projective_plane(n, check=True, existence=False):
r"""
Return a projective plane of order ``n`` as a 2-design.
A finite projective plane is a 2-design with `n^2+n+1` lines (or blocks) and
`n^2+n+1` points. For more information on finite projective planes, see the
:wikipedia:`Projective_plane#Finite_projective_planes`.
If no construction is possible, then the function raises a ``EmptySetError``
whereas if no construction is available the function raises a
``NotImplementedError``.
INPUT:
- ``n`` -- the finite projective plane's order
EXAMPLES::
sage: designs.projective_plane(2)
(7,3,1)-Balanced Incomplete Block Design
sage: designs.projective_plane(3)
(13,4,1)-Balanced Incomplete Block Design
sage: designs.projective_plane(4)
(21,5,1)-Balanced Incomplete Block Design
sage: designs.projective_plane(5)
(31,6,1)-Balanced Incomplete Block Design
sage: designs.projective_plane(6)
Traceback (most recent call last):
...
EmptySetError: By the Bruck-Ryser theorem, no projective plane of order 6 exists.
sage: designs.projective_plane(10)
Traceback (most recent call last):
...
EmptySetError: No projective plane of order 10 exists by C. Lam, L. Thiel and S. Swiercz "The nonexistence of finite projective planes of order 10" (1989), Canad. J. Math.
sage: designs.projective_plane(12)
Traceback (most recent call last):
...
NotImplementedError: If such a projective plane exists, we do not know how to build it.
sage: designs.projective_plane(14)
Traceback (most recent call last):
...
EmptySetError: By the Bruck-Ryser theorem, no projective plane of order 14 exists.
TESTS::
sage: designs.projective_plane(2197, existence=True)
True
sage: designs.projective_plane(6, existence=True)
False
sage: designs.projective_plane(10, existence=True)
False
sage: designs.projective_plane(12, existence=True)
Unknown
"""
from sage.rings.sum_of_squares import is_sum_of_two_squares_pyx
if n <= 1:
if existence:
return False
raise EmptySetError("There is no projective plane of order <= 1")
if n == 10:
if existence:
return False
ref = ("C. Lam, L. Thiel and S. Swiercz \"The nonexistence of finite "
"projective planes of order 10\" (1989), Canad. J. Math.")
raise EmptySetError("No projective plane of order 10 exists by %s"%ref)
if (n%4) in [1,2] and not is_sum_of_two_squares_pyx(n):
if existence:
return False
raise EmptySetError("By the Bruck-Ryser theorem, no projective"
" plane of order {} exists.".format(n))
if not is_prime_power(n):
if existence:
return Unknown
raise NotImplementedError("If such a projective plane exists, we do "
"not know how to build it.")
if existence:
return True
else:
return DesarguesianProjectivePlaneDesign(n, point_coordinates=False, check=check)
def AhrensSzekeresGeneralizedQuadrangleGraph(q, dual=False):
r"""
Return the collinearity graph of the generalized quadrangle `AS(q)`, or of its dual
Let `q` be an odd prime power. `AS(q)` is a generalized quadrangle [GQwiki]_ of
order `(q-1,q+1)`, see 3.1.5 in [PT09]_. Its points are elements
of `F_q^3`, and lines are sets of size `q` of the form
* `\{ (\sigma, a, b) \mid \sigma\in F_q \}`
* `\{ (a, \sigma, b) \mid \sigma\in F_q \}`
* `\{ (c \sigma^2 - b \sigma + a, -2 c \sigma + b, \sigma) \mid \sigma\in F_q \}`,
where `a`, `b`, `c` are arbitrary elements of `F_q`.
INPUT:
- ``q`` -- a power of an odd prime number
- ``dual`` -- if ``False`` (default), return the collinearity graph of `AS(q)`.
Otherwise return the collinearity graph of the dual `AS(q)`.
EXAMPLES::
sage: g=graphs.AhrensSzekeresGeneralizedQuadrangleGraph(5); g
AS(5); GQ(4, 6): Graph on 125 vertices
sage: g.is_strongly_regular(parameters=True)
(125, 28, 3, 7)
sage: g=graphs.AhrensSzekeresGeneralizedQuadrangleGraph(5,dual=True); g
AS(5)*; GQ(6, 4): Graph on 175 vertices
sage: g.is_strongly_regular(parameters=True)
(175, 30, 5, 5)
REFERENCE:
.. [GQwiki] `Generalized quadrangle
<http://en.wikipedia.org/wiki/Generalized_quadrangle>`__
.. [PT09] \S. Payne, J. A. Thas.
Finite generalized quadrangles.
European Mathematical Society,
2nd edition, 2009.
"""
from sage.combinat.designs.incidence_structures import IncidenceStructure
p, k = is_prime_power(q,get_data=True)
if k==0 or p==2:
raise ValueError('q must be an odd prime power')
F = FiniteField(q, 'a')
L = []
for a in F:
for b in F:
L.append(tuple(map(lambda s: (s, a, b), F)))
L.append(tuple(map(lambda s: (a, s, b), F)))
for c in F:
L.append(tuple(map(lambda s: (c*s**2 - b*s + a, -2*c*s + b, s), F)))
if dual:
G = IncidenceStructure(L).intersection_graph()
G.name('AS('+str(q)+')*; GQ'+str((q+1,q-1)))
else:
G = IncidenceStructure(L).dual().intersection_graph()
G.name('AS('+str(q)+'); GQ'+str((q-1,q+1)))
return G
def rshcd_from_prime_power_and_conference_matrix(n):
r"""
Return a `((n-1)^2,1)`-RSHCD if `n` is prime power, and symmetric `(n-1)`-conference matrix exists
The construction implemented here is Theorem 16 (and Corollary 17) from [WW72]_.
In [SWW72]_ this construction (Theorem 5.15 and Corollary 5.16)
is reproduced with a typo. Note that [WW72]_ refers to [Sz69]_ for the construction,
provided by :func:`szekeres_difference_set_pair`,
of complementary difference sets, and the latter has a typo.
From a :func:`symmetric_conference_matrix`, we only need the Seidel
adjacency matrix of the underlying strongly regular conference (i.e. Paley
type) graph, which we construct directly.
INPUT:
- ``n`` -- an integer
.. SEEALSO::
:func:`regular_symmetric_hadamard_matrix_with_constant_diagonal`
EXAMPLES:
A 36x36 example ::
sage: from sage.combinat.matrices.hadamard_matrix import rshcd_from_prime_power_and_conference_matrix
sage: from sage.combinat.matrices.hadamard_matrix import is_hadamard_matrix
sage: H = rshcd_from_prime_power_and_conference_matrix(7); H
36 x 36 dense matrix over Integer Ring (use the '.str()' method to see the entries)
sage: H==H.T and is_hadamard_matrix(H) and H.diagonal()==[1]*36 and list(sum(H))==[6]*36
True
Bigger examples, only provided by this construction ::
sage: H = rshcd_from_prime_power_and_conference_matrix(27) # long time
sage: H == H.T and is_hadamard_matrix(H) # long time
True
sage: H.diagonal()==[1]*676 and list(sum(H))==[26]*676 # long time
True
In this example the conference matrix is not Paley, as 45 is not a prime power ::
sage: H = rshcd_from_prime_power_and_conference_matrix(47) # not tested (long time)
REFERENCE:
.. [WW72] \J. Wallis and A.L. Whiteman,
Some classes of Hadamard matrices with constant diagonal,
Bull. Austral. Math. Soc. 7(1972), 233-249
"""
from sage.graphs.strongly_regular_db import strongly_regular_graph as srg
if is_prime_power(n) and 2==(n-1)%4:
try:
M = srg(n-2,(n-3)//2,(n-7)//4)
except ValueError:
return
m = (n-3)//4
Q,X,Y = szekeres_difference_set_pair(m)
B = typeI_matrix_difference_set(Q,X)
A = -typeI_matrix_difference_set(Q,Y) # must be symmetric
W = M.seidel_adjacency_matrix()
f = J(1,4*m+1)
e = J(1,2*m+1)
JJ = J(2*m+1, 2*m+1)
II = I(n-2)
Ib = I(2*m+1)
J4m = J(4*m+1,4*m+1)
H34 = -(B+Ib).tensor_product(W)+Ib.tensor_product(J4m)+(Ib-JJ).tensor_product(II)
A_t_W = A.tensor_product(W)
e_t_f = e.tensor_product(f)
H = block_matrix([
[J(1,1), f, e_t_f, -e_t_f],
[f.T, J4m, e.tensor_product(W-II), e.tensor_product(W+II)],
[ e_t_f.T, (e.T).tensor_product(W-II), A_t_W+JJ.tensor_product(II), H34],
[-e_t_f.T, (e.T).tensor_product(W+II), H34.T, -A_t_W+JJ.tensor_product(II)]])
return H
def benchmark(pbound=[3, 2**10],
nbound=[3, 2**8],
cbound=[1, Infinity],
obound=[1, Infinity],
loops=10,
tloop=Infinity,
tmax=Infinity,
prime=False,
even=False,
check=False,
fname=None,
write=False,
overwrite=False,
verbose=True,
skip_pari=False,
skip_magma=False,
skip_rains=False,
skip_kummer=False):
if write:
mode = 'w' if overwrite else 'a'
f = open(fname, mode, 0)
else:
f = sys.stdout
pmin, pmax = pbound
nmin, nmax = nbound
omin, omax = obound
cmin, cmax = cbound
M = Magma()
for p in xrange(pmin, pmax):
p = ZZ(p)
if not p.is_prime():
continue
for n in xrange(nmin, nmax):
n = ZZ(n)
if (prime == 1 and not is_prime(n)) or (prime == 2
and not is_prime_power(n)):
continue
if n < 2:
continue
if n % p == 0:
continue
if (not even) and (n % 2 == 0):
continue
o, G = find_root_order(p, [n, n], n, verbose=False)
m = G[0][0].parent().order()
c = Mod(p, n).multiplicative_order()
if verbose:
sys.stdout.write("\r" + " " * 79)
print("\rp = {}, n = {}, (o = {}, c = {})".format(p, n, o, c))
if verbose:
t = mytime()
sys.stdout.write("Constructing fields ({})".format(
time.strftime("%c")))
sys.stdout.flush()
q = p**n
k = GF(q, name='z')
k_rand = GF(q, modulus='random', name='z')
k_flint = GF_flint(p, k.modulus(), name='z')
if verbose > 1:
sys.stdout.write("\ntotal: {}s\n".format(mytime(t)))
sys.stdout.flush()
# Magma
if verbose:
sys.stdout.write("\r" + " " * 79)
sys.stdout.write("\rMagma ({})".format(time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_magma:
break
if (o > omax) or (o == p):
break
# let's assume that launching a new Magma instance is cheaper
# than computing random irreducible polynomials
try:
M._start()
except OSError as err:
# but it can also cause fork issues...
# let's accept this
# and fail as the situation will only worsen
# unless it is "just" a memory issue
# which should be mitigated by COW but is not
#print(err)
if err.errno == errno.ENOMEM:
break
else:
raise
try:
k_magma = M(k)
k_rand_magma = M(k_rand)
if tloop is not Infinity:
alarm(tloop)
t = mytime()
k_magma.Embed(k_rand_magma, nvals=0)
#M._eval_line("Embed(k_magma, k_rand_magma);", wait_for_prompt=False)
tloops += mytime(t)
except TypeError:
# sage/magma interface sometimes gets confused
pass
except (KeyboardInterrupt, AlarmInterrupt):
# sage interface eats KeyboardInterrupt
# and AlarmInterrupt derives from it
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
M.quit()
# sage pexpect interface leaves zombies around
try:
while os.waitpid(-1, os.WNOHANG)[0]:
pass
# but sometimes every child is already buried
# and we get an ECHILD error...
except OSError:
pass
if tloops > tmax:
break
tmagma = tloops / (l + 1)
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(
tloops, tloops / (l + 1)))
sys.stdout.flush()
# Rains algorithms
if verbose:
sys.stdout.write("\r" + " " * 79)
sys.stdout.write("\rCyclotomic Rains ({})".format(
time.strftime("%c")))
sys.stdout.flush()
trains = []
tloops = 0
for l in xrange(loops):
if skip_rains:
break
if (o > omax) or (o == p):
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_cyclorains(k_flint,
k_flint,
use_lucas=False)
tloops += mytime(t)
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
trains.append(tloops / (l + 1))
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(
tloops, tloops / (l + 1)))
sys.stdout.flush()
# Conic Rains
if verbose:
sys.stdout.write("\r" + " " * 79)
sys.stdout.write("\rConic Rains ({})".format(
time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_rains:
break
if (o != 2) or (o > omax) or (o == p):
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_cyclorains(k_flint,
k_flint,
use_lucas=True)
tloops += mytime(t)
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
trains.append(tloops / (l + 1))
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(
tloops, tloops / (l + 1)))
sys.stdout.flush()
# Elliptic Rains
if verbose:
sys.stdout.write("\r" + " " * 79)
sys.stdout.write("\rElliptic Rains ({})".format(
time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_rains:
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_ellrains(k_flint, k_flint)
tloops += mytime(t)
except RuntimeError:
# sometimes no suitable elliptic curve exists
pass
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
trains.append(tloops / (l + 1))
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(
tloops, tloops / (l + 1)))
sys.stdout.flush()
# PARI/GP
if verbose:
sys.stdout.write("\r" + " " * 79)
sys.stdout.write("\rPARI/GP ({})".format(time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_pari:
break
if c < cmin or c > cmax:
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_pari(k, k)
tloops += mytime(t)
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
tpari = tloops / (l + 1)
# Kummer algorithms
tkummer = []
# only linalg and modcomp implemented for c==1
for i, algo in enumerate(kummer_algolist[2 * (c == 1):-2 *
(c == 1) - 1]):
if verbose:
sys.stdout.write("\r" + " " * 79)
sys.stdout.write("\rKummer {} ({})".format(
kummer_namelist[2 * (c == 1) + i],
time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_kummer:
break
if c < cmin or c > cmax:
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_kummer(k_flint, k_flint, n, algo)
tloops += mytime(t)
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
tkummer.append(tloops / (l + 1))
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(
tloops, tloops / (l + 1)))
sys.stdout.flush()
tkummer = 2 * (c == 1) * [0] + tkummer + 2 * (c == 1) * [0]
if verbose:
sys.stdout.write("\r")
sys.stdout.flush()
f.write(("{} {} ({}, {})" + " {}" + " {}" * len(trains) + " {}" +
" {}" * len(tkummer) + "\n").format(
p, n, o, c, *([tmagma] + trains + [tpari] + tkummer)))
if write:
f.close()
def GDD_4_2(q, existence=False, check=True):
r"""
Return a `(2q,\{4\},\{2\})`-GDD for `q` a prime power with `q\equiv 1\pmod{6}`.
This method implements Lemma VII.5.17 from [BJL99] (p.495).
INPUT:
- ``q`` (integer)
- ``existence`` (boolean) -- instead of building the design, return:
- ``True`` -- meaning that Sage knows how to build the design
- ``Unknown`` -- meaning that Sage does not know how to build the
design, but that the design may exist (see :mod:`sage.misc.unknown`).
- ``False`` -- meaning that the design does not exist.
- ``check`` -- (boolean) Whether to check that output is correct before
returning it. As this is expected to be useless (but we are cautious
guys), you may want to disable it whenever you want speed. Set to ``True``
by default.
EXAMPLE::
sage: from sage.combinat.designs.group_divisible_designs import GDD_4_2
sage: GDD_4_2(7,existence=True)
True
sage: GDD_4_2(7)
Group Divisible Design on 14 points of type 2^7
sage: GDD_4_2(8,existence=True)
Unknown
sage: GDD_4_2(8)
Traceback (most recent call last):
...
NotImplementedError
"""
if q <= 1 or q % 6 != 1 or not is_prime_power(q):
if existence:
return Unknown
raise NotImplementedError
if existence:
return True
from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
G = GF(q, "x")
w = G.primitive_element()
e = w ** ((q - 1) // 3)
# A first parallel class is defined. G acts on it, which yields all others.
first_class = [[(0, 0), (1, w ** i), (1, e * w ** i), (1, e * e * w ** i)] for i in range((q - 1) // 6)]
label = {p: i for i, p in enumerate(G)}
classes = [[[2 * label[x[1] + g] + (x[0] + j) % 2 for x in S] for S in first_class] for g in G for j in range(2)]
return GroupDivisibleDesign(
2 * q,
groups=[[i, i + 1] for i in range(0, 2 * q, 2)],
blocks=sum(classes, []),
K=[4],
G=[2],
check=check,
copy=False,
)
def v_4_1_rbibd(v, existence=False):
r"""
Return a `(v,4,1)`-RBIBD.
INPUT:
- `n` (integer)
- ``existence`` (boolean; ``False`` by default) -- whether to build the
design or only answer whether it exists.
.. SEEALSO::
- :meth:`IncidenceStructure.is_resolvable`
- :func:`resolvable_balanced_incomplete_block_design`
.. NOTE::
A resolvable `(v,4,1)`-BIBD exists whenever `1\equiv 4\pmod(12)`. This
function, however, only implements a construction of `(v,4,1)`-BIBD such
that `v=3q+1\equiv 1\pmod{3}` where `q` is a prime power (see VII.7.5.a
from [BJL99]_).
EXAMPLE::
sage: rBIBD = designs.resolvable_balanced_incomplete_block_design(28,4)
sage: rBIBD.is_resolvable()
True
sage: rBIBD.is_t_design(return_parameters=True)
(True, (2, 28, 4, 1))
TESTS::
sage: for q in prime_powers(2,30):
....: if (3*q+1)%12 == 4:
....: _ = designs.resolvable_balanced_incomplete_block_design(3*q+1,4) # indirect doctest
"""
# Volume 1, VII.7.5.a from [BJL99]_
if v % 3 != 1 or not is_prime_power((v - 1) // 3):
if existence:
return Unknown
raise NotImplementedError(
"I don't know how to build a ({},{},1)-RBIBD!".format(v, 4))
from sage.rings.finite_rings.finite_field_constructor import FiniteField as GF
q = (v - 1) // 3
nn = (q - 1) // 4
G = GF(q, 'x')
w = G.primitive_element()
e = w**(nn)
assert e**2 == -1
first_class = [[(w**i, j), (-w**i, j), (e * w**i, j + 1),
(-e * w**i, j + 1)] for i in range(nn) for j in range(3)]
first_class.append([(0, 0), (0, 1), (0, 2), 'inf'])
label = {p: i for i, p in enumerate(G)}
classes = [[[
v - 1 if x == 'inf' else (x[1] % 3) * q + label[x[0] + g] for x in S
] for S in first_class] for g in G]
BIBD = BalancedIncompleteBlockDesign(v,
blocks=sum(classes, []),
k=4,
check=True,
copy=False)
BIBD._classes = classes
assert BIBD.is_resolvable()
return BIBD
def benchmark(pbound = [3, 2**10], nbound = [3, 2**8], cbound = [1, Infinity], obound = [1, Infinity], loops = 10, tloop = Infinity, tmax = Infinity, prime = False, even = False, check = False, fname = None, write = False, overwrite = False, verbose = True, skip_pari = False, skip_magma = False, skip_rains = False, skip_kummer = False):
if write:
mode = 'w' if overwrite else 'a'
f = open(fname, mode, 0)
else:
f = sys.stdout
pmin, pmax = pbound
nmin, nmax = nbound
omin, omax = obound
cmin, cmax = cbound
M = Magma()
for p in xrange(pmin, pmax):
p = ZZ(p)
if not p.is_prime():
continue
for n in xrange(nmin, nmax):
n = ZZ(n)
if (prime == 1 and not is_prime(n)) or (prime == 2 and not is_prime_power(n)):
continue
if n < 2:
continue
if n % p == 0:
continue
if (not even) and (n % 2 == 0):
continue
o, G = find_root_order(p, [n, n], n, verbose=False)
m = G[0][0].parent().order()
c = Mod(p,n).multiplicative_order()
if verbose:
sys.stdout.write("\r"+" "*79)
print("\rp = {}, n = {}, (o = {}, c = {})".format(p, n, o, c))
if verbose:
t = mytime()
sys.stdout.write("Constructing fields ({})".format(time.strftime("%c")))
sys.stdout.flush()
q = p**n
k = GF(q, name='z')
k_rand = GF(q, modulus='random', name='z')
k_flint = GF_flint(p, k.modulus(), name='z')
if verbose > 1:
sys.stdout.write("\ntotal: {}s\n".format(mytime(t)))
sys.stdout.flush()
# Magma
if verbose:
sys.stdout.write("\r"+" "*79)
sys.stdout.write("\rMagma ({})".format(time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_magma:
break
if (o > omax) or (o == p):
break
# let's assume that launching a new Magma instance is cheaper
# than computing random irreducible polynomials
try:
M._start()
except OSError as err:
# but it can also cause fork issues...
# let's accept this
# and fail as the situation will only worsen
# unless it is "just" a memory issue
# which should be mitigated by COW but is not
#print(err)
if err.errno == errno.ENOMEM:
break
else:
raise
try:
k_magma = M(k)
k_rand_magma = M(k_rand)
if tloop is not Infinity:
alarm(tloop)
t = mytime()
k_magma.Embed(k_rand_magma, nvals=0)
#M._eval_line("Embed(k_magma, k_rand_magma);", wait_for_prompt=False)
tloops += mytime(t)
except TypeError:
# sage/magma interface sometimes gets confused
pass
except (KeyboardInterrupt, AlarmInterrupt):
# sage interface eats KeyboardInterrupt
# and AlarmInterrupt derives from it
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
M.quit()
# sage pexpect interface leaves zombies around
try:
while os.waitpid(-1, os.WNOHANG)[0]:
pass
# but sometimes every child is already buried
# and we get an ECHILD error...
except OSError:
pass
if tloops > tmax:
break
tmagma = tloops / (l+1)
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(tloops, tloops/(l+1)))
sys.stdout.flush()
# Rains algorithms
if verbose:
sys.stdout.write("\r"+" "*79)
sys.stdout.write("\rCyclotomic Rains ({})".format(time.strftime("%c")))
sys.stdout.flush()
trains = []
tloops = 0
for l in xrange(loops):
if skip_rains:
break
if (o > omax) or (o == p):
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_cyclorains(k_flint, k_flint, use_lucas = False)
tloops += mytime(t)
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
trains.append(tloops / (l+1))
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(tloops, tloops/(l+1)))
sys.stdout.flush()
# Conic Rains
if verbose:
sys.stdout.write("\r"+" "*79)
sys.stdout.write("\rConic Rains ({})".format(time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_rains:
break
if (o != 2) or (o > omax) or (o == p):
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_cyclorains(k_flint, k_flint, use_lucas = True)
tloops += mytime(t)
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
trains.append(tloops / (l+1))
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(tloops, tloops/(l+1)))
sys.stdout.flush()
# Elliptic Rains
if verbose:
sys.stdout.write("\r"+" "*79)
sys.stdout.write("\rElliptic Rains ({})".format(time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_rains:
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_ellrains(k_flint, k_flint)
tloops += mytime(t)
except RuntimeError:
# sometimes no suitable elliptic curve exists
pass
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
trains.append(tloops / (l+1))
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(tloops, tloops/(l+1)))
sys.stdout.flush()
# PARI/GP
if verbose:
sys.stdout.write("\r"+" "*79)
sys.stdout.write("\rPARI/GP ({})".format(time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_pari:
break
if c < cmin or c > cmax:
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_pari(k, k)
tloops += mytime(t)
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
tpari = tloops / (l+1)
# Kummer algorithms
tkummer = []
# only linalg and modcomp implemented for c==1
for i, algo in enumerate(kummer_algolist[2*(c==1):-2*(c==1)-1]):
if verbose:
sys.stdout.write("\r"+" "*79)
sys.stdout.write("\rKummer {} ({})".format(kummer_namelist[2*(c==1)+i], time.strftime("%c")))
sys.stdout.flush()
tloops = 0
for l in xrange(loops):
if skip_kummer:
break
if c < cmin or c > cmax:
break
try:
if tloop is not Infinity:
alarm(tloop)
t = mytime()
a, b = find_gens_kummer(k_flint, k_flint, n, algo)
tloops += mytime(t)
except (KeyboardInterrupt, AlarmInterrupt):
tloops = 0
break
finally:
if tloop is not Infinity:
cancel_alarm()
if check and (l == 0 or check > 1):
g = a.minpoly()
if g.degree() != n:
raise RuntimeError("wrong degree")
if g != b.minpoly():
raise RuntimeError("different minpolys")
if tloops > tmax:
break
tkummer.append(tloops / (l+1))
if verbose > 1:
sys.stdout.write("\ntotal: {}s, per loop: {}s\n".format(tloops, tloops/(l+1)))
sys.stdout.flush()
tkummer = 2*(c == 1)*[0] + tkummer + 2*(c == 1)*[0]
if verbose:
sys.stdout.write("\r")
sys.stdout.flush()
f.write(("{} {} ({}, {})" + " {}" + " {}"*len(trains) + " {}" + " {}"*len(tkummer)+"\n").format(p, n, o, c, *([tmagma] + trains + [tpari] + tkummer)))
if write:
f.close()