def subsets(seq, k=None, repetition=False):
"""Generates all k-subsets (combinations) from an n-element set, seq.
A k-subset of an n-element set is any subset of length exactly k. The
number of k-subsets of an n-element set is given by binomial(n, k),
whereas there are 2**n subsets all together. If k is None then all
2**n subsets will be returned from shortest to longest.
Examples
========
>>> from sympy.utilities.iterables import subsets
subsets(seq, k) will return the n!/k!/(n - k)! k-subsets (combinations)
without repetition, i.e. once an item has been removed, it can no
longer be "taken":
>>> list(subsets([1, 2], 2))
[(1, 2)]
>>> list(subsets([1, 2]))
[(), (1,), (2,), (1, 2)]
>>> list(subsets([1, 2, 3], 2))
[(1, 2), (1, 3), (2, 3)]
subsets(seq, k, repetition=True) will return the (n - 1 + k)!/k!/(n - 1)!
combinations *with* repetition:
>>> list(subsets([1, 2], 2, repetition=True))
[(1, 1), (1, 2), (2, 2)]
If you ask for more items than are in the set you get the empty set unless
you allow repetitions:
>>> list(subsets([0, 1], 3, repetition=False))
[]
>>> list(subsets([0, 1], 3, repetition=True))
[(0, 0, 0), (0, 0, 1), (0, 1, 1), (1, 1, 1)]
"""
if k is None:
for k in range(len(seq) + 1):
for i in subsets(seq, k, repetition):
yield i
else:
if not repetition:
for i in combinations(seq, k):
yield i
else:
for i in combinations_with_replacement(seq, k):
yield i
def ksubsets(superset, k):
"""
Finds the subsets of size k in lexicographic order.
This uses the itertools generator.
Examples:
>>> from sympy.combinatorics.subsets import ksubsets
>>> list(ksubsets([1,2,3], 2))
[(1, 2), (1, 3), (2, 3)]
>>> list(ksubsets([1,2,3,4,5], 2))
[(1, 2), (1, 3), (1, 4), (1, 5), (2, 3), (2, 4), \
(2, 5), (3, 4), (3, 5), (4, 5)]
"""
return combinations(superset, k)
def ksubsets(superset, k):
"""
Finds the subsets of size k in lexicographic order.
This uses the itertools generator.
Examples
========
>>> from sympy.combinatorics.subsets import ksubsets
>>> list(ksubsets([1,2,3], 2))
[(1, 2), (1, 3), (2, 3)]
>>> list(ksubsets([1,2,3,4,5], 2))
[(1, 2), (1, 3), (1, 4), (1, 5), (2, 3), (2, 4), \
(2, 5), (3, 4), (3, 5), (4, 5)]
"""
return combinations(superset, k)
def sdm_groebner(G, NF, O, K):
"""
Compute a minimal standard basis of ``G`` with respect to order ``O``.
The algorithm uses a normal form ``NF``, for example ``sdm_nf_mora``.
The ground field is assumed to be ``K``, and monomials ordered according
to ``O``.
Let `N` denote the submodule generated by elements of `G`. A standard
basis for `N` is a subset `S` of `N`, such that `in(S) = in(N)`, where for
any subset `X` of `F`, `in(X)` denotes the submodule generated by the
initial forms of elements of `X`. [SCA, defn 2.3.2]
A standard basis is called minimal if no subset of it is a standard basis.
One may show that standard bases are always generating sets.
Minimal standard bases are not unique. This algorithm computes a
deterministic result, depending on the particular order of `G`.
See [SCA, algorithm 2.3.8, and remark 1.6.3].
"""
# First compute a standard basis
S = [f for f in G if f]
P = list(combinations(S, 2))
def prune(P, S, h):
"""
Prune the pair-set by applying the chain criterion
[SCA, remark 2.5.11].
"""
remove = set()
retain = set()
for (a, b, c) in permutations(S, 3):
A = sdm_LM(a)
B = sdm_LM(b)
C = sdm_LM(c)
if len(set([A[0], B[0], C[0]])) != 1 or not h in [a, b, c] or \
any(tuple(x) in retain for x in [a, b, c]):
continue
if monomial_divides(B[1:], monomial_lcm(A[1:], C[1:])):
remove.add((tuple(a), tuple(c)))
retain.update([tuple(b), tuple(c), tuple(a)])
return [(f, g) for (f, g) in P if (h not in [f, g]) or \
((tuple(f), tuple(g)) not in remove and \
(tuple(g), tuple(f)) not in remove)]
while P:
# TODO better data structures!!!
#print len(P), len(S)
# Use the "normal selection strategy"
lcms = [(i, sdm_LM(f)[:1] + monomial_lcm(sdm_LM(f)[1:], sdm_LM(g)[1:])) for \
i, (f, g) in enumerate(P)]
i = min(lcms, key=lambda x: O(x[1]))[0]
f, g = P.pop(i)
h = NF(sdm_spoly(f, g, O, K), S, O, K)
if h:
S.append(h)
P.extend((h, f) for f in S if sdm_LM(h)[0] == sdm_LM(f)[0])
P = prune(P, S, h)
# Now interreduce it. (TODO again, better data structures)
S = set(tuple(f) for f in S)
for a, b in permutations(S, 2):
A = sdm_LM(list(a))
B = sdm_LM(list(b))
if sdm_monomial_divides(A, B) and b in S and a in S:
S.remove(b)
return sorted((list(f) for f in S), key=lambda f: O(sdm_LM(f)),
reverse=True)
def sdm_groebner(G, NF, O, K):
"""
Compute a minimal standard basis of ``G`` with respect to order ``O``.
The algorithm uses a normal form ``NF``, for example ``sdm_nf_mora``.
The ground field is assumed to be ``K``, and monomials ordered according
to ``O``.
Let `N` denote the submodule generated by elements of `G`. A standard
basis for `N` is a subset `S` of `N`, such that `in(S) = in(N)`, where for
any subset `X` of `F`, `in(X)` denotes the submodule generated by the
initial forms of elements of `X`. [SCA, defn 2.3.2]
A standard basis is called minimal if no subset of it is a standard basis.
One may show that standard bases are always generating sets.
Minimal standard bases are not unique. This algorithm computes a
deterministic result, depending on the particular order of `G`.
See [SCA, algorithm 2.3.8, and remark 1.6.3].
"""
# First compute a standard basis
S = [f for f in G if f]
P = list(combinations(S, 2))
def prune(P, S, h):
"""
Prune the pair-set by applying the chain criterion
[SCA, remark 2.5.11].
"""
remove = set()
retain = set()
for (a, b, c) in permutations(S, 3):
A = sdm_LM(a)
B = sdm_LM(b)
C = sdm_LM(c)
if len(set([A[0], B[0], C[0]])) != 1 or not h in [a, b, c] or \
any(tuple(x) in retain for x in [a, b, c]):
continue
if monomial_divides(B[1:], monomial_lcm(A[1:], C[1:])):
remove.add((tuple(a), tuple(c)))
retain.update([tuple(b), tuple(c), tuple(a)])
return [(f, g) for (f, g) in P if (h not in [f, g]) or \
((tuple(f), tuple(g)) not in remove and \
(tuple(g), tuple(f)) not in remove)]
while P:
# TODO better data structures!!!
#print len(P), len(S)
# Use the "normal selection strategy"
lcms = [(i, sdm_LM(f)[:1] + monomial_lcm(sdm_LM(f)[1:], sdm_LM(g)[1:])) for \
i, (f, g) in enumerate(P)]
i = min(lcms, key=lambda x: O(x[1]))[0]
f, g = P.pop(i)
h = NF(sdm_spoly(f, g, O, K), S, O, K)
if h:
S.append(h)
P.extend((h, f) for f in S if sdm_LM(h)[0] == sdm_LM(f)[0])
P = prune(P, S, h)
# Now interreduce it. (TODO again, better data structures)
S = set(tuple(f) for f in S)
for a, b in permutations(S, 2):
A = sdm_LM(list(a))
B = sdm_LM(list(b))
if sdm_monomial_divides(A, B) and b in S and a in S:
S.remove(b)
return sorted((list(f) for f in S),
key=lambda f: O(sdm_LM(f)),
reverse=True)
