def cardinality(self):
"""
Returns the number of Lyndon words with the evaluation e.
EXAMPLES::
sage: LyndonWords([]).cardinality()
0
sage: LyndonWords([2,2]).cardinality()
1
sage: LyndonWords([2,3,2]).cardinality()
30
Check to make sure that the count matches up with the number of
Lyndon words generated.
::
sage: comps = [[],[2,2],[3,2,7],[4,2]] + Compositions(4).list()
sage: lws = [LyndonWords(comp) for comp in comps]
sage: all(lw.cardinality() == len(lw.list()) for lw in lws)
True
"""
evaluation = self._e
le = builtins.list(evaluation)
if len(evaluation) == 0:
return 0
n = sum(evaluation)
return sum([
moebius(j) * factorial(n / j) /
prod([factorial(ni / j) for ni in evaluation])
for j in divisors(gcd(le))
]) / n
def zero_eigenvectors(self, tg, flags):
if self._use_symmetry:
return self.symm_zero_eigenvectors(tg, flags)
cn = self._graph.n
s = tg.n
k = flags[0].n # assume all flags the same order
rows = []
for tv in Tuples(range(1, cn + 1), s):
it = self._graph.degenerate_induced_subgraph(tv)
using_phantom_edge = False
phantom_edge = None
if hasattr(self, "_phantom_edge") and it.ne == tg.ne - 1:
extra_edges = [e for e in tg if not e in it]
if len(extra_edges) == 1:
phantom_edge = extra_edges[0]
if all(tv[phantom_edge[i] - 1] == self._phantom_edge[i]
for i in range(tg.r)):
it.add_edge(phantom_edge)
using_phantom_edge = True
if not (using_phantom_edge or it.is_labelled_isomorphic(tg)):
continue
total = Integer(0)
row = [0] * len(flags)
for ov in UnorderedTuples(range(1, cn + 1), k - s):
factor = factorial(k - s)
for i in range(1, cn + 1):
factor /= factorial(ov.count(i))
if self._weights:
for v in ov:
factor *= self._weights[v - 1]
ig = self._graph.degenerate_induced_subgraph(tv + ov)
if using_phantom_edge:
ig.add_edge(phantom_edge)
ig.t = s
ig.make_minimal_isomorph()
for j in range(len(flags)):
if ig.is_labelled_isomorphic(flags[j]):
row[j] += factor
total += factor
break
for j in range(len(flags)):
row[j] /= total
rows.append(row)
return matrix_of_independent_rows(self._field, rows, len(flags))
def cardinality(self):
r"""
Return the cardinality of ``self``.
The number of ordered set partitions of a set of length `k` with
composition shape `\mu` is equal to
.. MATH::
\frac{k!}{\prod_{\mu_i \neq 0} \mu_i!}.
EXAMPLES::
sage: OrderedSetPartitions(5,[2,3]).cardinality()
10
sage: OrderedSetPartitions(0, []).cardinality()
1
sage: OrderedSetPartitions(0, [0]).cardinality()
1
sage: OrderedSetPartitions(0, [0,0]).cardinality()
1
sage: OrderedSetPartitions(5, [2,0,3]).cardinality()
10
"""
return factorial(len(self._set))/prod([factorial(i) for i in self.c])
def cardinality(self):
r"""
Return the cardinality of ``self``.
The number of ordered set partitions of a set of length `k` with
composition shape `\mu` is equal to
.. MATH::
\frac{k!}{\prod_{\mu_i \neq 0} \mu_i!}.
EXAMPLES::
sage: OrderedSetPartitions(5,[2,3]).cardinality()
10
sage: OrderedSetPartitions(0, []).cardinality()
1
sage: OrderedSetPartitions(0, [0]).cardinality()
1
sage: OrderedSetPartitions(0, [0,0]).cardinality()
1
sage: OrderedSetPartitions(5, [2,0,3]).cardinality()
10
"""
return factorial(len(self._set)) / prod([factorial(i) for i in self.c])
def cardinality(self):
"""
Returns the number of Lyndon words with the evaluation e.
EXAMPLES::
sage: LyndonWords([]).cardinality()
0
sage: LyndonWords([2,2]).cardinality()
1
sage: LyndonWords([2,3,2]).cardinality()
30
Check to make sure that the count matches up with the number of
Lyndon words generated.
::
sage: comps = [[],[2,2],[3,2,7],[4,2]] + Compositions(4).list()
sage: lws = [LyndonWords(comp) for comp in comps]
sage: all(lw.cardinality() == len(lw.list()) for lw in lws)
True
"""
evaluation = self._e
le = builtins.list(evaluation)
if len(evaluation) == 0:
return 0
n = sum(evaluation)
return sum([moebius(j)*factorial(n/j) / prod([factorial(ni/j) for ni in evaluation]) for j in divisors(gcd(le))])/n
def zero_eigenvectors(self, tg, flags):
if self._use_symmetry:
return self.symm_zero_eigenvectors(tg, flags)
cn = self._graph.n
s = tg.n
k = flags[0].n # assume all flags the same order
rows = []
for tv in Tuples(range(1, cn + 1), s):
it = self._graph.degenerate_induced_subgraph(tv)
using_phantom_edge = False
phantom_edge = None
if hasattr(self, "_phantom_edge") and it.ne == tg.ne - 1:
extra_edges = [e for e in tg if not e in it]
if len(extra_edges) == 1:
phantom_edge = extra_edges[0]
if all(tv[phantom_edge[i] - 1] == self._phantom_edge[i] for i in range(tg.r)):
it.add_edge(phantom_edge)
using_phantom_edge = True
if not (using_phantom_edge or it.is_labelled_isomorphic(tg)):
continue
total = Integer(0)
row = [0] * len(flags)
for ov in UnorderedTuples(range(1, cn + 1), k - s):
factor = factorial(k - s)
for i in range(1, cn + 1):
factor /= factorial(ov.count(i))
if self._weights:
for v in ov:
factor *= self._weights[v - 1]
ig = self._graph.degenerate_induced_subgraph(tv + ov)
if using_phantom_edge:
ig.add_edge(phantom_edge)
ig.t = s
ig.make_minimal_isomorph()
for j in range(len(flags)):
if ig.is_labelled_isomorphic(flags[j]):
row[j] += factor
total += factor
break
for j in range(len(flags)):
row[j] /= total
rows.append(row)
return matrix_of_independent_rows(self._field, rows, len(flags))
def by_taylor_expansion(self, fs, k, is_integral=False):
r"""
We combine the theta decomposition and the heat operator as in [Sko].
This yields a bijections of Jacobi forms of weight `k` and
`M_k \times S_{k+2} \times .. \times S_{k+2m}`.
"""
## we introduce an abbreviations
if is_integral:
PS = self.integral_power_series_ring()
else:
PS = self.power_series_ring()
if not len(fs) == self.__precision.jacobi_index() + 1:
raise ValueError(
"fs must be a list of m + 1 elliptic modular forms or their fourier expansion"
)
qexp_prec = self._qexp_precision()
if qexp_prec is None: # there are no forms below the precision
return dict()
f_divs = dict()
for (i, f) in enumerate(fs):
f_divs[(i, 0)] = PS(f(qexp_prec), qexp_prec)
if self.__precision.jacobi_index() == 1:
return self._by_taylor_expansion_m1(f_divs, k, is_integral)
for i in xrange(self.__precision.jacobi_index() + 1):
for j in xrange(1, self.__precision.jacobi_index() - i + 1):
f_divs[(i, j)] = f_divs[(i, j - 1)].derivative().shift(1)
phi_divs = list()
for i in xrange(self.__precision.jacobi_index() + 1):
## This is the formula in Skoruppas thesis. He uses d/ d tau instead of d / dz which yields
## a factor 4 m
phi_divs.append(
sum(f_divs[(j, i - j)] *
(4 * self.__precision.jacobi_index())**i * binomial(i, j) /
2**i #2**(self.__precision.jacobi_index() - i + 1)
* prod(2 * (i - l) + 1
for l in xrange(1, i)) / factorial(i + k + j - 1) *
factorial(2 * self.__precision.jacobi_index() + k - 1)
for j in xrange(i + 1)))
phi_coeffs = dict()
for r in xrange(self.__precision.jacobi_index() + 1):
series = sum(map(operator.mul, self._theta_factors()[r], phi_divs))
series = self._eta_factor() * series
for n in xrange(qexp_prec):
phi_coeffs[(n, r)] = series[n]
return phi_coeffs
def volume_hamming(n,q,r):
r"""
Returns number of elements in a Hamming ball of radius r in `\GF{q}^n`.
Agrees with Guava's SphereContent(n,r,GF(q)).
EXAMPLES::
sage: codes.bounds.volume_hamming(10,2,3)
176
"""
ans=sum([factorial(n)/(factorial(i)*factorial(n-i))*(q-1)**i for i in range(r+1)])
return ans
def _bracket_(self, right):
"""
The lambda bracket of these two elements.
The result is a dictionary with non-negative integer keys.
The value corresponding to the entry `j` is ``self_{(j)}right``.
EXAMPLES::
sage: Vir = lie_conformal_algebras.Virasoro(QQ); L = Vir.0
sage: L.bracket(L)
{0: TL, 1: 2*L, 3: 1/2*C}
sage: L.T().bracket(L)
{1: -TL, 2: -4*L, 4: -2*C}
sage: R = lie_conformal_algebras.Affine(QQbar, 'A1', names=('e','h','f')); R
The affine Lie conformal algebra of type ['A', 1] over Algebraic Field
sage: R.inject_variables()
Defining e, h, f, K
sage: e.bracket(f)
{0: h, 1: K}
sage: h.bracket(h.T())
{2: 4*K}
"""
p = self.parent()
if self.is_monomial() and right.is_monomial():
if self.is_zero() or right.is_zero():
return {}
s_coeff = p._s_coeff
a, k = self.index()
coefa = self.monomial_coefficients()[(a, k)]
b, m = right.index()
coefb = right.monomial_coefficients()[(b, m)]
try:
mbr = dict(s_coeff[(a, b)])
except KeyError:
return {}
pole = max(mbr.keys())
ret = {l: coefa*coefb*(-1)**k/factorial(k)*sum(factorial(l)\
/factorial(m+k+j-l)/factorial(l-k-j)/factorial(j)*\
mbr[j].T(m+k+j-l) for j in mbr if j >= l-m-k and\
j <= l-k) for l in range(m+k+pole+1)}
return {k: v for k, v in ret.items() if v}
diclist = [i._bracket_(j) for i in self.terms() for j in right.terms()]
ret = {}
pz = p.zero()
for d in diclist:
for k in d:
ret[k] = ret.get(k, pz) + d[k]
return {k: v for k, v in ret.items() if v}
def by_taylor_expansion(self, fs, k, is_integral=False) :
r"""
We combine the theta decomposition and the heat operator as in [Sko].
This yields a bijections of Jacobi forms of weight `k` and
`M_k \times S_{k+2} \times .. \times S_{k+2m}`.
"""
## we introduce an abbreviations
if is_integral :
PS = self.integral_power_series_ring()
else :
PS = self.power_series_ring()
if not len(fs) == self.__precision.jacobi_index() + 1 :
raise ValueError("fs must be a list of m + 1 elliptic modular forms or their fourier expansion")
qexp_prec = self._qexp_precision()
if qexp_prec is None : # there are no forms below the precision
return dict()
f_divs = dict()
for (i, f) in enumerate(fs) :
f_divs[(i, 0)] = PS(f(qexp_prec), qexp_prec)
if self.__precision.jacobi_index() == 1 :
return self._by_taylor_expansion_m1(f_divs, k, is_integral)
for i in xrange(self.__precision.jacobi_index() + 1) :
for j in xrange(1, self.__precision.jacobi_index() - i + 1) :
f_divs[(i,j)] = f_divs[(i, j - 1)].derivative().shift(1)
phi_divs = list()
for i in xrange(self.__precision.jacobi_index() + 1) :
## This is the formula in Skoruppas thesis. He uses d/ d tau instead of d / dz which yields
## a factor 4 m
phi_divs.append( sum( f_divs[(j, i - j)] * (4 * self.__precision.jacobi_index())**i
* binomial(i,j) / 2**i#2**(self.__precision.jacobi_index() - i + 1)
* prod(2*(i - l) + 1 for l in xrange(1, i))
/ factorial(i + k + j - 1)
* factorial(2*self.__precision.jacobi_index() + k - 1)
for j in xrange(i + 1) ) )
phi_coeffs = dict()
for r in xrange(self.__precision.jacobi_index() + 1) :
series = sum( map(operator.mul, self._theta_factors()[r], phi_divs) )
series = self._eta_factor() * series
for n in xrange(qexp_prec) :
phi_coeffs[(n, r)] = series[n]
return phi_coeffs
def volume_hamming(n,q,r):
r"""
Returns the number of elements in a Hamming ball.
Returns the number of elements in a Hamming ball of radius `r` in
`\GF{q}^n`.
EXAMPLES::
sage: codes.bounds.volume_hamming(10,2,3)
176
"""
ans=sum([factorial(n)/(factorial(i)*factorial(n-i))*(q-1)**i for i in range(r+1)])
return ans
def volume_hamming(n, q, r):
r"""
Returns number of elements in a Hamming ball of radius r in `\GF{q}^n`.
Agrees with Guava's SphereContent(n,r,GF(q)).
EXAMPLES::
sage: codes.bounds.volume_hamming(10,2,3)
176
"""
ans = sum([
factorial(n) / (factorial(i) * factorial(n - i)) * (q - 1)**i
for i in range(r + 1)
])
return ans
def subgraph_densities(self, n):
if n < 0:
raise ValueError
if self._use_symmetry:
return self.symm_subgraph_densities(n)
cn = self._graph.n
total = Integer(0)
sharp_graph_counts = {}
sharp_graphs = []
for P in UnorderedTuples(range(1, cn + 1), n):
factor = factorial(n)
for i in range(1, cn + 1):
factor /= factorial(P.count(i))
if self._weights:
for v in P:
factor *= self._weights[v - 1]
ig = self._graph.degenerate_induced_subgraph(P)
igc = copy(ig) # copy for phantom edge
ig.make_minimal_isomorph()
ghash = hash(ig)
if ghash in sharp_graph_counts:
sharp_graph_counts[ghash] += factor
else:
sharp_graphs.append(ig)
sharp_graph_counts[ghash] = factor
total += factor
if hasattr(self, "_phantom_edge") and all(
x in P for x in self._phantom_edge):
phantom_edge = [P.index(x) + 1 for x in self._phantom_edge]
igc.add_edge(phantom_edge)
igc.make_minimal_isomorph()
ghash = hash(igc)
if not ghash in sharp_graph_counts:
sharp_graphs.append(igc)
sharp_graph_counts[ghash] = Integer(0)
return [(g, sharp_graph_counts[hash(g)] / total) for g in sharp_graphs]
def f(partition):
n = 0
m = 1
for part in partition:
n += part
m *= factorial(part)
return t**n/m
def d(p):
r"""
Difference between the number of odd spin parity and even spin parity
standard labeled permutations with given profile
There is an explicit formula
.. MATH::
d(p) = \frac{(n-1)!}{2^{(n-k)/2}}
where `n` is the sum of the partition `p` and `k` is its length.
EXAMPLES::
sage: import surface_dynamics.interval_exchanges.rauzy_class_cardinality as rcc
sage: p = [3,3,1]
sage: rcc.d([3,3,1]) == factorial(6) / 2**2
True
sage: rcc.d([13]) == factorial(12) / 2**6
True
Adding marked points::
sage: p = [5,3,3]
sage: n = sum(p)
sage: all(rcc.d(p + [1]*k) == factorial(n+k-1) / factorial(n-1) * rcc.d(p) for k in xrange(1,6))
True
"""
n = sum(p)
k = len(p)
return factorial(n - 1) / 2**((n - k) / 2)
def d(p):
r"""
Difference between the number of odd spin parity and even spin parity
standard labeled permutations with given profile
There is an explicit formula
.. MATH::
d(p) = \frac{(n-1)!}{2^{(n-k)/2}}
where `n` is the sum of the partition `p` and `k` is its length.
EXAMPLES::
sage: import surface_dynamics.interval_exchanges.rauzy_class_cardinality as rcc
sage: p = [3,3,1]
sage: rcc.d([3,3,1]) == factorial(6) / 2**2
True
sage: rcc.d([13]) == factorial(12) / 2**6
True
Adding marked points::
sage: p = [5,3,3]
sage: n = sum(p)
sage: all(rcc.d(p + [1]*k) == factorial(n+k-1) / factorial(n-1) * rcc.d(p) for k in xrange(1,6))
True
"""
n = sum(p)
k = len(p)
return factorial(n-1) / 2**((n-k)/2)
def subgraph_densities(self, n):
if n < 0:
raise ValueError
if self._use_symmetry:
return self.symm_subgraph_densities(n)
cn = self._graph.n
total = Integer(0)
sharp_graph_counts = {}
sharp_graphs = []
for P in UnorderedTuples(range(1, cn + 1), n):
factor = factorial(n)
for i in range(1, cn + 1):
factor /= factorial(P.count(i))
if self._weights:
for v in P:
factor *= self._weights[v - 1]
ig = self._graph.degenerate_induced_subgraph(P)
igc = copy(ig) # copy for phantom edge
ig.make_minimal_isomorph()
ghash = hash(ig)
if ghash in sharp_graph_counts:
sharp_graph_counts[ghash] += factor
else:
sharp_graphs.append(ig)
sharp_graph_counts[ghash] = factor
total += factor
if hasattr(self, "_phantom_edge") and all(x in P for x in self._phantom_edge):
phantom_edge = [P.index(x) + 1 for x in self._phantom_edge]
igc.add_edge(phantom_edge)
igc.make_minimal_isomorph()
ghash = hash(igc)
if not ghash in sharp_graph_counts:
sharp_graphs.append(igc)
sharp_graph_counts[ghash] = Integer(0)
return [(g, sharp_graph_counts[hash(g)] / total) for g in sharp_graphs]
def quadratic_L_function__exact(n, d):
r"""
Returns the exact value of a quadratic twist of the Riemann Zeta function
by `\chi_d(x) = \left(\frac{d}{x}\right)`.
The input `n` must be a critical value.
EXAMPLES::
sage: quadratic_L_function__exact(1, -4)
1/4*pi
sage: quadratic_L_function__exact(-4, -4)
5/2
sage: quadratic_L_function__exact(2, 1)
1/6*pi^2
TESTS::
sage: quadratic_L_function__exact(2, -4)
Traceback (most recent call last):
...
TypeError: n must be a critical value (i.e. odd > 0 or even <= 0)
REFERENCES:
- [Iwa1972]_, pp 16-17, Special values of `L(1-n, \chi)` and `L(n, \chi)`
- [IR1990]_
- [Was1997]_
"""
from sage.all import SR, sqrt
if n <= 0:
return QuadraticBernoulliNumber(1 - n, d) / (n - 1)
elif n >= 1:
# Compute the kind of critical values (p10)
if kronecker_symbol(fundamental_discriminant(d), -1) == 1:
delta = 0
else:
delta = 1
# Compute the positive special values (p17)
if ((n - delta) % 2 == 0):
f = abs(fundamental_discriminant(d))
if delta == 0:
GS = sqrt(f)
else:
GS = I * sqrt(f)
ans = SR(ZZ(-1)**(1 + (n - delta) / 2))
ans *= (2 * pi / f)**n
ans *= GS # Evaluate the Gauss sum here! =0
ans *= QQ.one() / (2 * I**delta)
ans *= QuadraticBernoulliNumber(n, d) / factorial(n)
return ans
else:
if delta == 0:
raise TypeError(
"n must be a critical value (i.e. even > 0 or odd < 0)")
if delta == 1:
raise TypeError(
"n must be a critical value (i.e. odd > 0 or even <= 0)")
def _c_rec(p):
r"""
Recurrence function that is called by :func:`c`.
"""
if p[0] == 1: return factorial(len(p) - 1)
return (sum(_c_rec(split(p, k)) for k in xrange(1, p[0] - 1)) +
sum(p[i] * _c_rec(collapse(p, 0, i)) for i in xrange(1, len(p))))
def cardinality(self):
r"""
Return the number of integer necklaces with the evaluation ``content``.
The formula for the number of necklaces of content `\alpha`
a composition of `n` is:
.. MATH::
\sum_{d|gcd(\alpha)} \phi(d)
\binom{n/d}{\alpha_1/d, \ldots, \alpha_\ell/d},
where `\phi(d)` is the Euler `\phi` function.
EXAMPLES::
sage: Necklaces([]).cardinality()
0
sage: Necklaces([2,2]).cardinality()
2
sage: Necklaces([2,3,2]).cardinality()
30
sage: Necklaces([0,3,2]).cardinality()
2
Check to make sure that the count matches up with the number of
necklace words generated.
::
sage: comps = [[],[2,2],[3,2,7],[4,2],[0,4,2],[2,0,4]]+Compositions(4).list()
sage: ns = [ Necklaces(comp) for comp in comps]
sage: all( [ n.cardinality() == len(n.list()) for n in ns] )
True
"""
evaluation = self._content
le = list(evaluation)
if not le:
return 0
n = sum(le)
return sum(
euler_phi(j) * factorial(n / j) /
prod(factorial(ni / j) for ni in evaluation)
for j in divisors(gcd(le))) / n
def cardinality(self):
r"""
Return the number of integer necklaces with the evaluation ``content``.
The formula for the number of necklaces of content `\alpha`
a composition of `n` is:
.. MATH::
\sum_{d|gcd(\alpha)} \phi(d)
\binom{n/d}{\alpha_1/d, \ldots, \alpha_\ell/d},
where `\phi(d)` is the Euler `\phi` function.
EXAMPLES::
sage: Necklaces([]).cardinality()
0
sage: Necklaces([2,2]).cardinality()
2
sage: Necklaces([2,3,2]).cardinality()
30
sage: Necklaces([0,3,2]).cardinality()
2
Check to make sure that the count matches up with the number of
necklace words generated.
::
sage: comps = [[],[2,2],[3,2,7],[4,2],[0,4,2],[2,0,4]]+Compositions(4).list()
sage: ns = [Necklaces(comp) for comp in comps]
sage: all(n.cardinality() == len(n.list()) for n in ns)
True
"""
evaluation = self._content
le = list(evaluation)
if not le:
return ZZ.zero()
n = sum(le)
return ZZ.sum(euler_phi(j) * factorial(n // j) //
prod(factorial(ni // j) for ni in evaluation)
for j in divisors(gcd(le))) // n
def zeta__exact(n):
r"""
Returns the exact value of the Riemann Zeta function
The argument must be a critical value, namely either positive even
or negative odd.
See for example [Iwasawa]_, p13, Special value of `\zeta(2k)`
EXAMPLES:
Let us test the accuracy for negative special values::
sage: RR = RealField(100)
sage: for i in range(1,10):
... print "zeta(" + str(1-2*i) + "): ", RR(zeta__exact(1-2*i)) - zeta(RR(1-2*i))
zeta(-1): 0.00000000000000000000000000000
zeta(-3): 0.00000000000000000000000000000
zeta(-5): 0.00000000000000000000000000000
zeta(-7): 0.00000000000000000000000000000
zeta(-9): 0.00000000000000000000000000000
zeta(-11): 0.00000000000000000000000000000
zeta(-13): 0.00000000000000000000000000000
zeta(-15): 0.00000000000000000000000000000
zeta(-17): 0.00000000000000000000000000000
Let us test the accuracy for positive special values::
sage: all(abs(RR(zeta__exact(2*i))-zeta(RR(2*i))) < 10**(-28) for i in range(1,10))
True
TESTS::
sage: zeta__exact(5)
Traceback (most recent call last):
...
TypeError: n must be a critical value (i.e. even > 0 or odd < 0)
REFERENCES:
.. [Iwasawa] Iwasawa, *Lectures on p-adic L-functions*
.. [IreRos] Ireland and Rosen, *A Classical Introduction to Modern Number Theory*
.. [WashCyc] Washington, *Cyclotomic Fields*
"""
if n < 0:
return bernoulli(1 - n) / (n - 1)
elif n > 1:
if (n % 2 == 0):
return ZZ(-1)**(n / 2 + 1) * ZZ(2)**(
n - 1) * pi**n * bernoulli(n) / factorial(n)
else:
raise TypeError(
"n must be a critical value (i.e. even > 0 or odd < 0)")
elif n == 1:
return infinity
elif n == 0:
return -1 / 2
def quadratic_L_function__exact(n, d):
r"""
Returns the exact value of a quadratic twist of the Riemann Zeta function
by `\chi_d(x) = \left(\frac{d}{x}\right)`.
The input `n` must be a critical value.
EXAMPLES::
sage: quadratic_L_function__exact(1, -4)
1/4*pi
sage: quadratic_L_function__exact(-4, -4)
5/2
sage: quadratic_L_function__exact(2, 1)
1/6*pi^2
TESTS::
sage: quadratic_L_function__exact(2, -4)
Traceback (most recent call last):
...
TypeError: n must be a critical value (i.e. odd > 0 or even <= 0)
REFERENCES:
- [Iwa1972]_, pp 16-17, Special values of `L(1-n, \chi)` and `L(n, \chi)`
- [IR1990]_
- [Was1997]_
"""
from sage.all import SR, sqrt
if n <= 0:
return QuadraticBernoulliNumber(1-n,d)/(n-1)
elif n >= 1:
# Compute the kind of critical values (p10)
if kronecker_symbol(fundamental_discriminant(d), -1) == 1:
delta = 0
else:
delta = 1
# Compute the positive special values (p17)
if ((n - delta) % 2 == 0):
f = abs(fundamental_discriminant(d))
if delta == 0:
GS = sqrt(f)
else:
GS = I * sqrt(f)
ans = SR(ZZ(-1)**(1+(n-delta)/2))
ans *= (2*pi/f)**n
ans *= GS # Evaluate the Gauss sum here! =0
ans *= QQ.one()/(2 * I**delta)
ans *= QuadraticBernoulliNumber(n,d)/factorial(n)
return ans
else:
if delta == 0:
raise TypeError("n must be a critical value (i.e. even > 0 or odd < 0)")
if delta == 1:
raise TypeError("n must be a critical value (i.e. odd > 0 or even <= 0)")
def _c_rec(p):
r"""
Recurrence function that is called by :func:`c`.
"""
if p[0] == 1: return factorial(len(p)-1)
return (
sum(_c_rec(split(p,k)) for k in xrange(1,p[0]-1)) +
sum(p[i]*_c_rec(collapse(p,0,i)) for i in xrange(1,len(p))))
def cardinality(self):
r"""
Return the cardinality of ``self``.
The number of fully packed loops on `n \times n` grid
.. MATH::
\prod_{k=0}^{n-1} \frac{(3k+1)!}{(n+k)!} = \frac{1! 4! 7! 10!
\cdots (3n-2)!}{n! (n+1)! (n+2)! (n+3)! \cdots (2n-1)!}.
EXAMPLES::
sage: [AlternatingSignMatrices(n).cardinality() for n in range(0, 11)]
[1, 1, 2, 7, 42, 429, 7436, 218348, 10850216, 911835460, 129534272700]
"""
return Integer(prod( [ factorial(3*k+1)/factorial(self._n+k)
for k in range(self._n)] ))
def cardinality(self):
r"""
Return the cardinality of ``self``.
The number of decorated permutations of size `n` is equal to
.. MATH::
\sum_{k=0^n} \frac{n!}{k!}
EXAMPLES::
sage: [DecoratedPermutations(n).cardinality() for n in range(11)]
[1, 2, 5, 16, 65, 326, 1957, 13700, 109601, 986410, 9864101]
"""
return Integer(
sum(
factorial(self._n) // factorial(k)
for k in range(self._n + 1)))
def zeta__exact(n):
r"""
Returns the exact value of the Riemann Zeta function
The argument must be a critical value, namely either positive even
or negative odd.
See for example [Iwasawa]_, p13, Special value of `\zeta(2k)`
EXAMPLES:
Let us test the accuracy for negative special values::
sage: RR = RealField(100)
sage: for i in range(1,10):
... print "zeta(" + str(1-2*i) + "): ", RR(zeta__exact(1-2*i)) - zeta(RR(1-2*i))
zeta(-1): 0.00000000000000000000000000000
zeta(-3): 0.00000000000000000000000000000
zeta(-5): 0.00000000000000000000000000000
zeta(-7): 0.00000000000000000000000000000
zeta(-9): 0.00000000000000000000000000000
zeta(-11): 0.00000000000000000000000000000
zeta(-13): 0.00000000000000000000000000000
zeta(-15): 0.00000000000000000000000000000
zeta(-17): 0.00000000000000000000000000000
Let us test the accuracy for positive special values::
sage: all(abs(RR(zeta__exact(2*i))-zeta(RR(2*i))) < 10**(-28) for i in range(1,10))
True
TESTS::
sage: zeta__exact(5)
Traceback (most recent call last):
...
TypeError: n must be a critical value (i.e. even > 0 or odd < 0)
REFERENCES:
.. [Iwasawa] Iwasawa, *Lectures on p-adic L-functions*
.. [IreRos] Ireland and Rosen, *A Classical Introduction to Modern Number Theory*
.. [WashCyc] Washington, *Cyclotomic Fields*
"""
if n < 0:
return bernoulli(1-n)/(n-1)
elif n > 1:
if (n % 2 == 0):
return ZZ(-1)**(n/2 + 1) * ZZ(2)**(n-1) * pi**n * bernoulli(n) / factorial(n)
else:
raise TypeError("n must be a critical value (i.e. even > 0 or odd < 0)")
elif n==1:
return infinity
elif n==0:
return -1/2
def cardinality(self):
"""
Return the cardinality of ``self``.
The number of ordered partitions of a set of size `n` into `k`
parts is equal to `k! S(n,k)` where `S(n,k)` denotes the Stirling
number of the second kind.
EXAMPLES::
sage: OrderedSetPartitions(4,2).cardinality()
14
sage: OrderedSetPartitions(4,1).cardinality()
1
"""
return factorial(self.n) * stirling_number2(len(self._set), self.n)
def cardinality(self):
"""
Return the cardinality of ``self``.
The number of ordered partitions of a set of size `n` into `k`
parts is equal to `k! S(n,k)` where `S(n,k)` denotes the Stirling
number of the second kind.
EXAMPLES::
sage: OrderedSetPartitions(4,2).cardinality()
14
sage: OrderedSetPartitions(4,1).cardinality()
1
"""
return factorial(self.n)*stirling_number2(len(self._set), self.n)
def cardinality(self):
"""
EXAMPLES::
sage: OrderedSetPartitions(0).cardinality()
1
sage: OrderedSetPartitions(1).cardinality()
1
sage: OrderedSetPartitions(2).cardinality()
3
sage: OrderedSetPartitions(3).cardinality()
13
sage: OrderedSetPartitions([1,2,3]).cardinality()
13
sage: OrderedSetPartitions(4).cardinality()
75
sage: OrderedSetPartitions(5).cardinality()
541
"""
return sum([factorial(k)*stirling_number2(len(self._set),k) for k in range(len(self._set)+1)])
def cardinality(self):
"""
EXAMPLES::
sage: OrderedSetPartitions(0).cardinality()
1
sage: OrderedSetPartitions(1).cardinality()
1
sage: OrderedSetPartitions(2).cardinality()
3
sage: OrderedSetPartitions(3).cardinality()
13
sage: OrderedSetPartitions([1,2,3]).cardinality()
13
sage: OrderedSetPartitions(4).cardinality()
75
sage: OrderedSetPartitions(5).cardinality()
541
"""
return sum([factorial(k)*stirling_number2(len(self._set),k) for k in range(len(self._set)+1)])
def expansion_on_basis(self, w):
r"""
Return the expansion of `S_w` in words of the shuffle algebra.
INPUT:
- ``w`` -- a word
EXAMPLES::
sage: S = ShuffleAlgebra(QQ, 'ab').dual_pbw_basis()
sage: S.expansion_on_basis(Word())
B[word: ]
sage: S.expansion_on_basis(Word()).parent()
Shuffle Algebra on 2 generators ['a', 'b'] over Rational Field
sage: S.expansion_on_basis(Word('abba'))
2*B[word: aabb] + B[word: abab] + B[word: abba]
sage: S.expansion_on_basis(Word())
B[word: ]
sage: S.expansion_on_basis(Word('abab'))
2*B[word: aabb] + B[word: abab]
"""
from sage.arith.all import factorial
if not w:
return self._alg.one()
if len(w) == 1:
return self._alg.monomial(w)
if w.is_lyndon():
W = self.basis().keys()
letter = W([w[0]])
expansion = self.expansion_on_basis(W(w[1:]))
return self._alg.sum_of_terms(
(letter * i, c) for i, c in expansion)
lf = w.lyndon_factorization()
powers = {}
for i in lf:
powers[i] = powers.get(i, 0) + 1
denom = prod(factorial(p) for p in powers.values())
result = self._alg.prod(self.expansion_on_basis(i) for i in lf)
return self._alg(result / denom)
def tuple_orbit_reps(self, k, prefix=None):
if prefix is None:
prefix = []
s = len(prefix)
tp = tuple(prefix)
if s > k:
raise ValueError
if k == 0:
return 1, {(): 1}
gens = self._graph.automorphism_group_gens()
# Pass generators to GAP to create a group for us.
gen_str = ",".join("(" + "".join(str(cy) for cy in cys) + ")"
for cys in gens)
gap.eval("g := Group(%s);" % gen_str)
if len(prefix) > 0:
gap.eval("g := Stabilizer(g, %s, OnTuples);" % list(set(prefix)))
S = []
for i in range(1, k - s + 1):
S.extend(
[tuple(sorted(list(x))) for x in Subsets(self._graph.n, i)])
set_orb_reps = {}
#sys.stdout.write("Calculating orbits")
while len(S) > 0:
rep = list(S[0])
o = gap.new("Orbit(g, %s, OnSets);" % (rep, )).sage()
o = list(set([tuple(sorted(t)) for t in o]))
ot = o[0]
set_orb_reps[ot] = len(o)
for t in o:
S.remove(t)
#sys.stdout.write(".")
#sys.stdout.flush()
#sys.stdout.write("\n")
combs = [tuple(c) for c in Compositions(k - s)]
factors = []
for c in combs:
factor = factorial(k - s)
for x in c:
factor /= factorial(x)
factors.append(factor)
orb_reps = {}
total = 0
for ot, length in set_orb_reps.iteritems():
ne = len(ot)
for ci in range(len(combs)):
c = combs[ci]
if len(c) == ne:
t = tp
for i in range(ne):
t += c[i] * (ot[i], )
weight = factors[ci] * length
orb_reps[t] = weight
total += weight
return total, orb_reps
def gamma__exact(n):
r"""
Evaluates the exact value of the `\Gamma` function at an integer or
half-integer argument.
EXAMPLES::
sage: gamma__exact(4)
6
sage: gamma__exact(3)
2
sage: gamma__exact(2)
1
sage: gamma__exact(1)
1
sage: gamma__exact(1/2)
sqrt(pi)
sage: gamma__exact(3/2)
1/2*sqrt(pi)
sage: gamma__exact(5/2)
3/4*sqrt(pi)
sage: gamma__exact(7/2)
15/8*sqrt(pi)
sage: gamma__exact(-1/2)
-2*sqrt(pi)
sage: gamma__exact(-3/2)
4/3*sqrt(pi)
sage: gamma__exact(-5/2)
-8/15*sqrt(pi)
sage: gamma__exact(-7/2)
16/105*sqrt(pi)
TESTS::
sage: gamma__exact(1/3)
Traceback (most recent call last):
...
TypeError: you must give an integer or half-integer argument
"""
from sage.all import sqrt
n = QQ(n)
if denominator(n) == 1:
if n <= 0:
return infinity
if n > 0:
return factorial(n - 1)
elif denominator(n) == 2:
ans = QQ.one()
while n != QQ((1, 2)):
if n < 0:
ans /= n
n += 1
elif n > 0:
n += -1
ans *= n
ans *= sqrt(pi)
return ans
else:
raise TypeError("you must give an integer or half-integer argument")
def splitting_field(poly, name, map=False, degree_multiple=None, abort_degree=None, simplify=True, simplify_all=False):
"""
Compute the splitting field of a given polynomial, defined over a
number field.
INPUT:
- ``poly`` -- a monic polynomial over a number field
- ``name`` -- a variable name for the number field
- ``map`` -- (default: ``False``) also return an embedding of
``poly`` into the resulting field. Note that computing this
embedding might be expensive.
- ``degree_multiple`` -- a multiple of the absolute degree of
the splitting field. If ``degree_multiple`` equals the actual
degree, this can enormously speed up the computation.
- ``abort_degree`` -- abort by raising a :class:`SplittingFieldAbort`
if it can be determined that the absolute degree of the splitting
field is strictly larger than ``abort_degree``.
- ``simplify`` -- (default: ``True``) during the algorithm, try
to find a simpler defining polynomial for the intermediate
number fields using PARI's ``polred()``. This usually speeds
up the computation but can also considerably slow it down.
Try and see what works best in the given situation.
- ``simplify_all`` -- (default: ``False``) If ``True``, simplify
intermediate fields and also the resulting number field.
OUTPUT:
If ``map`` is ``False``, the splitting field as an absolute number
field. If ``map`` is ``True``, a tuple ``(K, phi)`` where ``phi``
is an embedding of the base field in ``K``.
EXAMPLES::
sage: R.<x> = PolynomialRing(QQ)
sage: K.<a> = (x^3 + 2).splitting_field(); K
Number Field in a with defining polynomial x^6 + 3*x^5 + 6*x^4 + 11*x^3 + 12*x^2 - 3*x + 1
sage: K.<a> = (x^3 - 3*x + 1).splitting_field(); K
Number Field in a with defining polynomial x^3 - 3*x + 1
The ``simplify`` and ``simplify_all`` flags usually yield
fields defined by polynomials with smaller coefficients.
By default, ``simplify`` is True and ``simplify_all`` is False.
::
sage: (x^4 - x + 1).splitting_field('a', simplify=False)
Number Field in a with defining polynomial x^24 - 2780*x^22 + 2*x^21 + 3527512*x^20 - 2876*x^19 - 2701391985*x^18 + 945948*x^17 + 1390511639677*x^16 + 736757420*x^15 - 506816498313560*x^14 - 822702898220*x^13 + 134120588299548463*x^12 + 362240696528256*x^11 - 25964582366880639486*x^10 - 91743672243419990*x^9 + 3649429473447308439427*x^8 + 14310332927134072336*x^7 - 363192569823568746892571*x^6 - 1353403793640477725898*x^5 + 24293393281774560140427565*x^4 + 70673814899934142357628*x^3 - 980621447508959243128437933*x^2 - 1539841440617805445432660*x + 18065914012013502602456565991
sage: (x^4 - x + 1).splitting_field('a', simplify=True)
Number Field in a with defining polynomial x^24 + 8*x^23 - 32*x^22 - 310*x^21 + 540*x^20 + 4688*x^19 - 6813*x^18 - 32380*x^17 + 49525*x^16 + 102460*x^15 - 129944*x^14 - 287884*x^13 + 372727*x^12 + 150624*x^11 - 110530*x^10 - 566926*x^9 + 1062759*x^8 - 779940*x^7 + 863493*x^6 - 1623578*x^5 + 1759513*x^4 - 955624*x^3 + 459975*x^2 - 141948*x + 53919
sage: (x^4 - x + 1).splitting_field('a', simplify_all=True)
Number Field in a with defining polynomial x^24 - 3*x^23 + 2*x^22 - x^20 + 4*x^19 + 32*x^18 - 35*x^17 - 92*x^16 + 49*x^15 + 163*x^14 - 15*x^13 - 194*x^12 - 15*x^11 + 163*x^10 + 49*x^9 - 92*x^8 - 35*x^7 + 32*x^6 + 4*x^5 - x^4 + 2*x^2 - 3*x + 1
Reducible polynomials also work::
sage: pol = (x^4 - 1)*(x^2 + 1/2)*(x^2 + 1/3)
sage: pol.splitting_field('a', simplify_all=True)
Number Field in a with defining polynomial x^8 - x^4 + 1
Relative situation::
sage: R.<x> = PolynomialRing(QQ)
sage: K.<a> = NumberField(x^3 + 2)
sage: S.<t> = PolynomialRing(K)
sage: L.<b> = (t^2 - a).splitting_field()
sage: L
Number Field in b with defining polynomial t^6 + 2
With ``map=True``, we also get the embedding of the base field
into the splitting field::
sage: L.<b>, phi = (t^2 - a).splitting_field(map=True)
sage: phi
Ring morphism:
From: Number Field in a with defining polynomial x^3 + 2
To: Number Field in b with defining polynomial t^6 + 2
Defn: a |--> b^2
sage: (x^4 - x + 1).splitting_field('a', simplify_all=True, map=True)[1]
Ring morphism:
From: Rational Field
To: Number Field in a with defining polynomial x^24 - 3*x^23 + 2*x^22 - x^20 + 4*x^19 + 32*x^18 - 35*x^17 - 92*x^16 + 49*x^15 + 163*x^14 - 15*x^13 - 194*x^12 - 15*x^11 + 163*x^10 + 49*x^9 - 92*x^8 - 35*x^7 + 32*x^6 + 4*x^5 - x^4 + 2*x^2 - 3*x + 1
Defn: 1 |--> 1
We can enable verbose messages::
sage: set_verbose(2)
sage: K.<a> = (x^3 - x + 1).splitting_field()
verbose 1 (...: splitting_field.py, splitting_field) Starting field: y
verbose 1 (...: splitting_field.py, splitting_field) SplittingData to factor: [(3, 0)]
verbose 2 (...: splitting_field.py, splitting_field) Done factoring (time = ...)
verbose 1 (...: splitting_field.py, splitting_field) SplittingData to handle: [(2, 2), (3, 3)]
verbose 1 (...: splitting_field.py, splitting_field) Bounds for absolute degree: [6, 6]
verbose 2 (...: splitting_field.py, splitting_field) Handling polynomial x^2 + 23
verbose 1 (...: splitting_field.py, splitting_field) New field before simplifying: x^2 + 23 (time = ...)
verbose 1 (...: splitting_field.py, splitting_field) New field: y^2 - y + 6 (time = ...)
verbose 2 (...: splitting_field.py, splitting_field) Converted polynomials to new field (time = ...)
verbose 1 (...: splitting_field.py, splitting_field) SplittingData to factor: []
verbose 2 (...: splitting_field.py, splitting_field) Done factoring (time = ...)
verbose 1 (...: splitting_field.py, splitting_field) SplittingData to handle: [(3, 3)]
verbose 1 (...: splitting_field.py, splitting_field) Bounds for absolute degree: [6, 6]
verbose 2 (...: splitting_field.py, splitting_field) Handling polynomial x^3 - x + 1
verbose 1 (...: splitting_field.py, splitting_field) New field: y^6 + 3*y^5 + 19*y^4 + 35*y^3 + 127*y^2 + 73*y + 271 (time = ...)
sage: set_verbose(0)
Try all Galois groups in degree 4. We use a quadratic base field
such that ``polgalois()`` cannot be used::
sage: R.<x> = PolynomialRing(QuadraticField(-11))
sage: C2C2pol = x^4 - 10*x^2 + 1
sage: C2C2pol.splitting_field('x')
Number Field in x with defining polynomial x^8 + 24*x^6 + 608*x^4 + 9792*x^2 + 53824
sage: C4pol = x^4 + x^3 + x^2 + x + 1
sage: C4pol.splitting_field('x')
Number Field in x with defining polynomial x^8 - x^7 - 2*x^6 + 5*x^5 + x^4 + 15*x^3 - 18*x^2 - 27*x + 81
sage: D8pol = x^4 - 2
sage: D8pol.splitting_field('x')
Number Field in x with defining polynomial x^16 + 8*x^15 + 68*x^14 + 336*x^13 + 1514*x^12 + 5080*x^11 + 14912*x^10 + 35048*x^9 + 64959*x^8 + 93416*x^7 + 88216*x^6 + 41608*x^5 - 25586*x^4 - 60048*x^3 - 16628*x^2 + 12008*x + 34961
sage: A4pol = x^4 - 4*x^3 + 14*x^2 - 28*x + 21
sage: A4pol.splitting_field('x')
Number Field in x with defining polynomial x^24 - 20*x^23 + 290*x^22 - 3048*x^21 + 26147*x^20 - 186132*x^19 + 1130626*x^18 - 5913784*x^17 + 26899345*x^16 - 106792132*x^15 + 371066538*x^14 - 1127792656*x^13 + 2991524876*x^12 - 6888328132*x^11 + 13655960064*x^10 - 23000783036*x^9 + 32244796382*x^8 - 36347834476*x^7 + 30850889884*x^6 - 16707053128*x^5 + 1896946429*x^4 + 4832907884*x^3 - 3038258802*x^2 - 200383596*x + 593179173
sage: S4pol = x^4 + x + 1
sage: S4pol.splitting_field('x')
Number Field in x with defining polynomial x^48 ...
Some bigger examples::
sage: R.<x> = PolynomialRing(QQ)
sage: pol15 = chebyshev_T(31, x) - 1 # 2^30*(x-1)*minpoly(cos(2*pi/31))^2
sage: pol15.splitting_field('a')
Number Field in a with defining polynomial x^15 - x^14 - 14*x^13 + 13*x^12 + 78*x^11 - 66*x^10 - 220*x^9 + 165*x^8 + 330*x^7 - 210*x^6 - 252*x^5 + 126*x^4 + 84*x^3 - 28*x^2 - 8*x + 1
sage: pol48 = x^6 - 4*x^4 + 12*x^2 - 12
sage: pol48.splitting_field('a')
Number Field in a with defining polynomial x^48 ...
If you somehow know the degree of the field in advance, you
should add a ``degree_multiple`` argument. This can speed up the
computation, in particular for polynomials of degree >= 12 or
for relative extensions::
sage: pol15.splitting_field('a', degree_multiple=15)
Number Field in a with defining polynomial x^15 + x^14 - 14*x^13 - 13*x^12 + 78*x^11 + 66*x^10 - 220*x^9 - 165*x^8 + 330*x^7 + 210*x^6 - 252*x^5 - 126*x^4 + 84*x^3 + 28*x^2 - 8*x - 1
A value for ``degree_multiple`` which isn't actually a
multiple of the absolute degree of the splitting field can
either result in a wrong answer or the following exception::
sage: pol48.splitting_field('a', degree_multiple=20)
Traceback (most recent call last):
...
ValueError: inconsistent degree_multiple in splitting_field()
Compute the Galois closure as the splitting field of the defining polynomial::
sage: R.<x> = PolynomialRing(QQ)
sage: pol48 = x^6 - 4*x^4 + 12*x^2 - 12
sage: K.<a> = NumberField(pol48)
sage: L.<b> = pol48.change_ring(K).splitting_field()
sage: L
Number Field in b with defining polynomial x^48 ...
Try all Galois groups over `\QQ` in degree 5 except for `S_5`
(the latter is infeasible with the current implementation)::
sage: C5pol = x^5 + x^4 - 4*x^3 - 3*x^2 + 3*x + 1
sage: C5pol.splitting_field('x')
Number Field in x with defining polynomial x^5 + x^4 - 4*x^3 - 3*x^2 + 3*x + 1
sage: D10pol = x^5 - x^4 - 5*x^3 + 4*x^2 + 3*x - 1
sage: D10pol.splitting_field('x')
Number Field in x with defining polynomial x^10 - 28*x^8 + 216*x^6 - 681*x^4 + 902*x^2 - 401
sage: AGL_1_5pol = x^5 - 2
sage: AGL_1_5pol.splitting_field('x')
Number Field in x with defining polynomial x^20 + 10*x^19 + 55*x^18 + 210*x^17 + 595*x^16 + 1300*x^15 + 2250*x^14 + 3130*x^13 + 3585*x^12 + 3500*x^11 + 2965*x^10 + 2250*x^9 + 1625*x^8 + 1150*x^7 + 750*x^6 + 400*x^5 + 275*x^4 + 100*x^3 + 75*x^2 + 25
sage: A5pol = x^5 - x^4 + 2*x^2 - 2*x + 2
sage: A5pol.splitting_field('x')
Number Field in x with defining polynomial x^60 ...
We can use the ``abort_degree`` option if we don't want to compute
fields of too large degree (this can be used to check whether the
splitting field has small degree)::
sage: (x^5+x+3).splitting_field('b', abort_degree=119)
Traceback (most recent call last):
...
SplittingFieldAbort: degree of splitting field equals 120
sage: (x^10+x+3).splitting_field('b', abort_degree=60) # long time (10s on sage.math, 2014)
Traceback (most recent call last):
...
SplittingFieldAbort: degree of splitting field is a multiple of 180
Use the ``degree_divisor`` attribute to recover the divisor of the
degree of the splitting field or ``degree_multiple`` to recover a
multiple::
sage: from sage.rings.number_field.splitting_field import SplittingFieldAbort
sage: try: # long time (4s on sage.math, 2014)
....: (x^8+x+1).splitting_field('b', abort_degree=60, simplify=False)
....: except SplittingFieldAbort as e:
....: print(e.degree_divisor)
....: print(e.degree_multiple)
120
1440
TESTS::
sage: from sage.rings.number_field.splitting_field import splitting_field
sage: splitting_field(polygen(QQ), name='x', map=True, simplify_all=True)
(Number Field in x with defining polynomial x, Ring morphism:
From: Rational Field
To: Number Field in x with defining polynomial x
Defn: 1 |--> 1)
"""
from sage.misc.all import verbose, cputime
degree_multiple = Integer(degree_multiple or 0)
abort_degree = Integer(abort_degree or 0)
# Kpol = PARI polynomial in y defining the extension found so far
F = poly.base_ring()
if is_RationalField(F):
Kpol = pari("'y")
else:
Kpol = F.pari_polynomial("y")
# Fgen = the generator of F as element of Q[y]/Kpol
# (only needed if map=True)
if map:
Fgen = F.gen().__pari__()
verbose("Starting field: %s"%Kpol)
# L and Lred are lists of SplittingData.
# L contains polynomials which are irreducible over K,
# Lred contains polynomials which need to be factored.
L = []
Lred = [SplittingData(poly._pari_with_name(), degree_multiple)]
# Main loop, handle polynomials one by one
while True:
# Absolute degree of current field K
absolute_degree = Integer(Kpol.poldegree())
# Compute minimum relative degree of splitting field
rel_degree_divisor = Integer(1)
for splitting in L:
rel_degree_divisor = rel_degree_divisor.lcm(splitting.poldegree())
# Check for early aborts
abort_rel_degree = abort_degree//absolute_degree
if abort_rel_degree and rel_degree_divisor > abort_rel_degree:
raise SplittingFieldAbort(absolute_degree * rel_degree_divisor, degree_multiple)
# First, factor polynomials in Lred and store the result in L
verbose("SplittingData to factor: %s"%[s._repr_tuple() for s in Lred])
t = cputime()
for splitting in Lred:
m = splitting.dm.gcd(degree_multiple).gcd(factorial(splitting.poldegree()))
if m == 1:
continue
factors = Kpol.nffactor(splitting.pol)[0]
for q in factors:
d = q.poldegree()
fac = factorial(d)
# Multiple of the degree of the splitting field of q,
# note that the degree equals fac iff the Galois group is S_n.
mq = m.gcd(fac)
if mq == 1:
continue
# Multiple of the degree of the splitting field of q
# over the field defined by adding square root of the
# discriminant.
# If the Galois group is contained in A_n, then mq_alt is
# also the degree multiple over the current field K.
# Here, we have equality if the Galois group is A_n.
mq_alt = mq.gcd(fac//2)
# If we are over Q, then use PARI's polgalois() to compute
# these degrees exactly.
if absolute_degree == 1:
try:
G = q.polgalois()
except PariError:
pass
else:
mq = Integer(G[0])
mq_alt = mq//2 if (G[1] == -1) else mq
# In degree 4, use the cubic resolvent to refine the
# degree bounds.
if d == 4 and mq >= 12: # mq equals 12 or 24
# Compute cubic resolvent
a0, a1, a2, a3, a4 = (q/q.pollead()).Vecrev()
assert a4 == 1
cubicpol = pari([4*a0*a2 - a1*a1 -a0*a3*a3, a1*a3 - 4*a0, -a2, 1]).Polrev()
cubicfactors = Kpol.nffactor(cubicpol)[0]
if len(cubicfactors) == 1: # A4 or S4
# After adding a root of the cubic resolvent,
# the degree of the extension defined by q
# is a factor 3 smaller.
L.append(SplittingData(cubicpol, 3))
rel_degree_divisor = rel_degree_divisor.lcm(3)
mq = mq//3 # 4 or 8
mq_alt = 4
elif len(cubicfactors) == 2: # C4 or D8
# The irreducible degree 2 factor is
# equivalent to x^2 - q.poldisc().
discpol = cubicfactors[1]
L.append(SplittingData(discpol, 2))
mq = mq_alt = 4
else: # C2 x C2
mq = mq_alt = 4
if mq > mq_alt >= 3:
# Add quadratic resolvent x^2 - D to decrease
# the degree multiple by a factor 2.
discpol = pari([-q.poldisc(), 0, 1]).Polrev()
discfactors = Kpol.nffactor(discpol)[0]
if len(discfactors) == 1:
# Discriminant is not a square
L.append(SplittingData(discpol, 2))
rel_degree_divisor = rel_degree_divisor.lcm(2)
mq = mq_alt
L.append(SplittingData(q, mq))
rel_degree_divisor = rel_degree_divisor.lcm(q.poldegree())
if abort_rel_degree and rel_degree_divisor > abort_rel_degree:
raise SplittingFieldAbort(absolute_degree * rel_degree_divisor, degree_multiple)
verbose("Done factoring", t, level=2)
if len(L) == 0: # Nothing left to do
break
# Recompute absolute degree multiple
new_degree_multiple = absolute_degree
for splitting in L:
new_degree_multiple *= splitting.dm
degree_multiple = new_degree_multiple.gcd(degree_multiple)
# Absolute degree divisor
degree_divisor = rel_degree_divisor * absolute_degree
# Sort according to degree to handle low degrees first
L.sort(key=lambda x: x.key())
verbose("SplittingData to handle: %s"%[s._repr_tuple() for s in L])
verbose("Bounds for absolute degree: [%s, %s]"%(degree_divisor,degree_multiple))
# Check consistency
if degree_multiple % degree_divisor != 0:
raise ValueError("inconsistent degree_multiple in splitting_field()")
for splitting in L:
# The degree of the splitting field must be a multiple of
# the degree of the polynomial. Only do this check for
# SplittingData with minimal dm, because the higher dm are
# defined as relative degree over the splitting field of
# the polynomials with lesser dm.
if splitting.dm > L[0].dm:
break
if splitting.dm % splitting.poldegree() != 0:
raise ValueError("inconsistent degree_multiple in splitting_field()")
# Add a root of f = L[0] to construct the field N = K[x]/f(x)
splitting = L[0]
f = splitting.pol
verbose("Handling polynomial %s"%(f.lift()), level=2)
t = cputime()
Npol, KtoN, k = Kpol.rnfequation(f, flag=1)
# Make Npol monic integral primitive, store in Mpol
# (after this, we don't need Npol anymore, only Mpol)
Mdiv = pari(1)
Mpol = Npol
while True:
denom = Integer(Mpol.pollead())
if denom == 1:
break
denom = pari(denom.factor().radical_value())
Mpol = (Mpol*(denom**Mpol.poldegree())).subst("x", pari([0,1/denom]).Polrev("x"))
Mpol /= Mpol.content()
Mdiv *= denom
# We are finished for sure if we hit the degree bound
finished = (Mpol.poldegree() >= degree_multiple)
if simplify_all or (simplify and not finished):
# Find a simpler defining polynomial Lpol for Mpol
verbose("New field before simplifying: %s"%Mpol, t)
t = cputime()
M = Mpol.polred(flag=3)
n = len(M[0])-1
Lpol = M[1][n].change_variable_name("y")
LtoM = M[0][n].change_variable_name("y").Mod(Mpol.change_variable_name("y"))
MtoL = LtoM.modreverse()
else:
# Lpol = Mpol
Lpol = Mpol.change_variable_name("y")
MtoL = pari("'y")
NtoL = MtoL/Mdiv
KtoL = KtoN.lift().subst("x", NtoL).Mod(Lpol)
Kpol = Lpol # New Kpol (for next iteration)
verbose("New field: %s"%Kpol, t)
if map:
t = cputime()
Fgen = Fgen.lift().subst("y", KtoL)
verbose("Computed generator of F in K", t, level=2)
if finished:
break
t = cputime()
# Convert f and elements of L from K to L and store in L
# (if the polynomial is certain to remain irreducible) or Lred.
Lold = L[1:]
L = []
Lred = []
# First add f divided by the linear factor we obtained,
# mg is the new degree multiple.
mg = splitting.dm//f.poldegree()
if mg > 1:
g = [c.subst("y", KtoL).Mod(Lpol) for c in f.Vecrev().lift()]
g = pari(g).Polrev()
g /= pari([k*KtoL - NtoL, 1]).Polrev() # divide linear factor
Lred.append(SplittingData(g, mg))
for splitting in Lold:
g = [c.subst("y", KtoL) for c in splitting.pol.Vecrev().lift()]
g = pari(g).Polrev()
mg = splitting.dm
if Integer(g.poldegree()).gcd(f.poldegree()) == 1: # linearly disjoint fields
L.append(SplittingData(g, mg))
else:
Lred.append(SplittingData(g, mg))
verbose("Converted polynomials to new field", t, level=2)
# Convert Kpol to Sage and construct the absolute number field
Kpol = PolynomialRing(RationalField(), name=poly.variable_name())(Kpol/Kpol.pollead())
K = NumberField(Kpol, name)
if map:
return K, F.hom(Fgen, K)
else:
return K
def _closed_form(hyp):
a, b, z = hyp.operands()
a, b = a.operands(), b.operands()
p, q = len(a), len(b)
if z == 0:
return Integer(1)
if p == q == 0:
return exp(z)
if p == 1 and q == 0:
return (1 - z) ** (-a[0])
if p == 0 and q == 1:
# TODO: make this require only linear time
def _0f1(b, z):
F12 = cosh(2 * sqrt(z))
F32 = sinh(2 * sqrt(z)) / (2 * sqrt(z))
if 2 * b == 1:
return F12
if 2 * b == 3:
return F32
if 2 * b > 3:
return ((b - 2) * (b - 1) / z * (_0f1(b - 2, z) -
_0f1(b - 1, z)))
if 2 * b < 1:
return (_0f1(b + 1, z) + z / (b * (b + 1)) *
_0f1(b + 2, z))
raise ValueError
# Can evaluate 0F1 in terms of elementary functions when
# the parameter is a half-integer
if 2 * b[0] in ZZ and b[0] not in ZZ:
return _0f1(b[0], z)
# Confluent hypergeometric function
if p == 1 and q == 1:
aa, bb = a[0], b[0]
if aa * 2 == 1 and bb * 2 == 3:
t = sqrt(-z)
return sqrt(pi) / 2 * erf(t) / t
if a == 1 and b == 2:
return (exp(z) - 1) / z
n, m = aa, bb
if n in ZZ and m in ZZ and m > 0 and n > 0:
rf = rising_factorial
if m <= n:
return (exp(z) * sum(rf(m - n, k) * (-z) ** k /
factorial(k) / rf(m, k) for k in
xrange(n - m + 1)))
else:
T = sum(rf(n - m + 1, k) * z ** k /
(factorial(k) * rf(2 - m, k)) for k in
xrange(m - n))
U = sum(rf(1 - n, k) * (-z) ** k /
(factorial(k) * rf(2 - m, k)) for k in
xrange(n))
return (factorial(m - 2) * rf(1 - m, n) *
z ** (1 - m) / factorial(n - 1) *
(T - exp(z) * U))
if p == 2 and q == 1:
R12 = QQ('1/2')
R32 = QQ('3/2')
def _2f1(a, b, c, z):
"""
Evaluation of 2F1(a, b; c; z), assuming a, b, c positive
integers or half-integers
"""
if b == c:
return (1 - z) ** (-a)
if a == c:
return (1 - z) ** (-b)
if a == 0 or b == 0:
return Integer(1)
if a > b:
a, b = b, a
if b >= 2:
F1 = _2f1(a, b - 1, c, z)
F2 = _2f1(a, b - 2, c, z)
q = (b - 1) * (z - 1)
return (((c - 2 * b + 2 + (b - a - 1) * z) * F1 +
(b - c - 1) * F2) / q)
if c > 2:
# how to handle this case?
if a - c + 1 == 0 or b - c + 1 == 0:
raise NotImplementedError
F1 = _2f1(a, b, c - 1, z)
F2 = _2f1(a, b, c - 2, z)
r1 = (c - 1) * (2 - c - (a + b - 2 * c + 3) * z)
r2 = (c - 1) * (c - 2) * (1 - z)
q = (a - c + 1) * (b - c + 1) * z
return (r1 * F1 + r2 * F2) / q
if (a, b, c) == (R12, 1, 2):
return (2 - 2 * sqrt(1 - z)) / z
if (a, b, c) == (1, 1, 2):
return -log(1 - z) / z
if (a, b, c) == (1, R32, R12):
return (1 + z) / (1 - z) ** 2
if (a, b, c) == (1, R32, 2):
return 2 * (1 / sqrt(1 - z) - 1) / z
if (a, b, c) == (R32, 2, R12):
return (1 + 3 * z) / (1 - z) ** 3
if (a, b, c) == (R32, 2, 1):
return (2 + z) / (2 * (sqrt(1 - z) * (1 - z) ** 2))
if (a, b, c) == (2, 2, 1):
return (1 + z) / (1 - z) ** 3
raise NotImplementedError
aa, bb = a
cc, = b
if z == 1:
return (gamma(cc) * gamma(cc - aa - bb) / gamma(cc - aa) /
gamma(cc - bb))
if ((aa * 2) in ZZ and (bb * 2) in ZZ and (cc * 2) in ZZ and
aa > 0 and bb > 0 and cc > 0):
try:
return _2f1(aa, bb, cc, z)
except NotImplementedError:
pass
return hyp
def tuple_orbit_reps(self, k, prefix=None):
if prefix is None:
prefix = []
s = len(prefix)
tp = tuple(prefix)
if s > k:
raise ValueError
if k == 0:
return 1, {() : 1}
gens = self._graph.automorphism_group_gens()
# Pass generators to GAP to create a group for us.
gen_str = ",".join("(" + "".join(str(cy) for cy in cys) + ")" for cys in gens)
gap.eval("g := Group(%s);" % gen_str)
if len(prefix) > 0:
gap.eval("g := Stabilizer(g, %s, OnTuples);" % list(set(prefix)))
S = []
for i in range(1, k - s + 1):
S.extend([tuple(sorted(list(x))) for x in Subsets(self._graph.n, i)])
set_orb_reps = {}
#sys.stdout.write("Calculating orbits")
while len(S) > 0:
rep = list(S[0])
o = gap.new("Orbit(g, %s, OnSets);" % (rep,)).sage()
o = list(set([tuple(sorted(t)) for t in o]))
ot = o[0]
set_orb_reps[ot] = len(o)
for t in o:
S.remove(t)
#sys.stdout.write(".")
#sys.stdout.flush()
#sys.stdout.write("\n")
combs = [tuple(c) for c in Compositions(k - s)]
factors = []
for c in combs:
factor = factorial(k - s)
for x in c:
factor /= factorial(x)
factors.append(factor)
orb_reps = {}
total = 0
for ot, length in set_orb_reps.iteritems():
ne = len(ot)
for ci in range(len(combs)):
c = combs[ci]
if len(c) == ne:
t = tp
for i in range(ne):
t += c[i] * (ot[i],)
weight = factors[ci] * length
orb_reps[t] = weight
total += weight
return total, orb_reps
def splitting_field(poly,
name,
map=False,
degree_multiple=None,
abort_degree=None,
simplify=True,
simplify_all=False):
r"""
Compute the splitting field of a given polynomial, defined over a
number field.
INPUT:
- ``poly`` -- a monic polynomial over a number field
- ``name`` -- a variable name for the number field
- ``map`` -- (default: ``False``) also return an embedding of
``poly`` into the resulting field. Note that computing this
embedding might be expensive.
- ``degree_multiple`` -- a multiple of the absolute degree of
the splitting field. If ``degree_multiple`` equals the actual
degree, this can enormously speed up the computation.
- ``abort_degree`` -- abort by raising a :class:`SplittingFieldAbort`
if it can be determined that the absolute degree of the splitting
field is strictly larger than ``abort_degree``.
- ``simplify`` -- (default: ``True``) during the algorithm, try
to find a simpler defining polynomial for the intermediate
number fields using PARI's ``polred()``. This usually speeds
up the computation but can also considerably slow it down.
Try and see what works best in the given situation.
- ``simplify_all`` -- (default: ``False``) If ``True``, simplify
intermediate fields and also the resulting number field.
OUTPUT:
If ``map`` is ``False``, the splitting field as an absolute number
field. If ``map`` is ``True``, a tuple ``(K, phi)`` where ``phi``
is an embedding of the base field in ``K``.
EXAMPLES::
sage: R.<x> = PolynomialRing(QQ)
sage: K.<a> = (x^3 + 2).splitting_field(); K
Number Field in a with defining polynomial x^6 + 3*x^5 + 6*x^4 + 11*x^3 + 12*x^2 - 3*x + 1
sage: K.<a> = (x^3 - 3*x + 1).splitting_field(); K
Number Field in a with defining polynomial x^3 - 3*x + 1
The ``simplify`` and ``simplify_all`` flags usually yield
fields defined by polynomials with smaller coefficients.
By default, ``simplify`` is True and ``simplify_all`` is False.
::
sage: (x^4 - x + 1).splitting_field('a', simplify=False)
Number Field in a with defining polynomial x^24 - 2780*x^22 + 2*x^21 + 3527512*x^20 - 2876*x^19 - 2701391985*x^18 + 945948*x^17 + 1390511639677*x^16 + 736757420*x^15 - 506816498313560*x^14 - 822702898220*x^13 + 134120588299548463*x^12 + 362240696528256*x^11 - 25964582366880639486*x^10 - 91743672243419990*x^9 + 3649429473447308439427*x^8 + 14310332927134072336*x^7 - 363192569823568746892571*x^6 - 1353403793640477725898*x^5 + 24293393281774560140427565*x^4 + 70673814899934142357628*x^3 - 980621447508959243128437933*x^2 - 1539841440617805445432660*x + 18065914012013502602456565991
sage: (x^4 - x + 1).splitting_field('a', simplify=True)
Number Field in a with defining polynomial x^24 + 8*x^23 - 32*x^22 - 310*x^21 + 540*x^20 + 4688*x^19 - 6813*x^18 - 32380*x^17 + 49525*x^16 + 102460*x^15 - 129944*x^14 - 287884*x^13 + 372727*x^12 + 150624*x^11 - 110530*x^10 - 566926*x^9 + 1062759*x^8 - 779940*x^7 + 863493*x^6 - 1623578*x^5 + 1759513*x^4 - 955624*x^3 + 459975*x^2 - 141948*x + 53919
sage: (x^4 - x + 1).splitting_field('a', simplify_all=True)
Number Field in a with defining polynomial x^24 - 3*x^23 + 2*x^22 - x^20 + 4*x^19 + 32*x^18 - 35*x^17 - 92*x^16 + 49*x^15 + 163*x^14 - 15*x^13 - 194*x^12 - 15*x^11 + 163*x^10 + 49*x^9 - 92*x^8 - 35*x^7 + 32*x^6 + 4*x^5 - x^4 + 2*x^2 - 3*x + 1
Reducible polynomials also work::
sage: pol = (x^4 - 1)*(x^2 + 1/2)*(x^2 + 1/3)
sage: pol.splitting_field('a', simplify_all=True)
Number Field in a with defining polynomial x^8 - x^4 + 1
Relative situation::
sage: R.<x> = PolynomialRing(QQ)
sage: K.<a> = NumberField(x^3 + 2)
sage: S.<t> = PolynomialRing(K)
sage: L.<b> = (t^2 - a).splitting_field()
sage: L
Number Field in b with defining polynomial t^6 + 2
With ``map=True``, we also get the embedding of the base field
into the splitting field::
sage: L.<b>, phi = (t^2 - a).splitting_field(map=True)
sage: phi
Ring morphism:
From: Number Field in a with defining polynomial x^3 + 2
To: Number Field in b with defining polynomial t^6 + 2
Defn: a |--> b^2
sage: (x^4 - x + 1).splitting_field('a', simplify_all=True, map=True)[1]
Ring morphism:
From: Rational Field
To: Number Field in a with defining polynomial x^24 - 3*x^23 + 2*x^22 - x^20 + 4*x^19 + 32*x^18 - 35*x^17 - 92*x^16 + 49*x^15 + 163*x^14 - 15*x^13 - 194*x^12 - 15*x^11 + 163*x^10 + 49*x^9 - 92*x^8 - 35*x^7 + 32*x^6 + 4*x^5 - x^4 + 2*x^2 - 3*x + 1
Defn: 1 |--> 1
We can enable verbose messages::
sage: from sage.misc.verbose import set_verbose
sage: set_verbose(2)
sage: K.<a> = (x^3 - x + 1).splitting_field()
verbose 1 (...: splitting_field.py, splitting_field) Starting field: y
verbose 1 (...: splitting_field.py, splitting_field) SplittingData to factor: [(3, 0)]
verbose 2 (...: splitting_field.py, splitting_field) Done factoring (time = ...)
verbose 1 (...: splitting_field.py, splitting_field) SplittingData to handle: [(2, 2), (3, 3)]
verbose 1 (...: splitting_field.py, splitting_field) Bounds for absolute degree: [6, 6]
verbose 2 (...: splitting_field.py, splitting_field) Handling polynomial x^2 + 23
verbose 1 (...: splitting_field.py, splitting_field) New field before simplifying: x^2 + 23 (time = ...)
verbose 1 (...: splitting_field.py, splitting_field) New field: y^2 - y + 6 (time = ...)
verbose 2 (...: splitting_field.py, splitting_field) Converted polynomials to new field (time = ...)
verbose 1 (...: splitting_field.py, splitting_field) SplittingData to factor: []
verbose 2 (...: splitting_field.py, splitting_field) Done factoring (time = ...)
verbose 1 (...: splitting_field.py, splitting_field) SplittingData to handle: [(3, 3)]
verbose 1 (...: splitting_field.py, splitting_field) Bounds for absolute degree: [6, 6]
verbose 2 (...: splitting_field.py, splitting_field) Handling polynomial x^3 - x + 1
verbose 1 (...: splitting_field.py, splitting_field) New field: y^6 + 3*y^5 + 19*y^4 + 35*y^3 + 127*y^2 + 73*y + 271 (time = ...)
sage: set_verbose(0)
Try all Galois groups in degree 4. We use a quadratic base field
such that ``polgalois()`` cannot be used::
sage: R.<x> = PolynomialRing(QuadraticField(-11))
sage: C2C2pol = x^4 - 10*x^2 + 1
sage: C2C2pol.splitting_field('x')
Number Field in x with defining polynomial x^8 + 24*x^6 + 608*x^4 + 9792*x^2 + 53824
sage: C4pol = x^4 + x^3 + x^2 + x + 1
sage: C4pol.splitting_field('x')
Number Field in x with defining polynomial x^8 - x^7 - 2*x^6 + 5*x^5 + x^4 + 15*x^3 - 18*x^2 - 27*x + 81
sage: D8pol = x^4 - 2
sage: D8pol.splitting_field('x')
Number Field in x with defining polynomial x^16 + 8*x^15 + 68*x^14 + 336*x^13 + 1514*x^12 + 5080*x^11 + 14912*x^10 + 35048*x^9 + 64959*x^8 + 93416*x^7 + 88216*x^6 + 41608*x^5 - 25586*x^4 - 60048*x^3 - 16628*x^2 + 12008*x + 34961
sage: A4pol = x^4 - 4*x^3 + 14*x^2 - 28*x + 21
sage: A4pol.splitting_field('x')
Number Field in x with defining polynomial x^24 - 20*x^23 + 290*x^22 - 3048*x^21 + 26147*x^20 - 186132*x^19 + 1130626*x^18 - 5913784*x^17 + 26899345*x^16 - 106792132*x^15 + 371066538*x^14 - 1127792656*x^13 + 2991524876*x^12 - 6888328132*x^11 + 13655960064*x^10 - 23000783036*x^9 + 32244796382*x^8 - 36347834476*x^7 + 30850889884*x^6 - 16707053128*x^5 + 1896946429*x^4 + 4832907884*x^3 - 3038258802*x^2 - 200383596*x + 593179173
sage: S4pol = x^4 + x + 1
sage: S4pol.splitting_field('x')
Number Field in x with defining polynomial x^48 ...
Some bigger examples::
sage: R.<x> = PolynomialRing(QQ)
sage: pol15 = chebyshev_T(31, x) - 1 # 2^30*(x-1)*minpoly(cos(2*pi/31))^2
sage: pol15.splitting_field('a')
Number Field in a with defining polynomial x^15 - x^14 - 14*x^13 + 13*x^12 + 78*x^11 - 66*x^10 - 220*x^9 + 165*x^8 + 330*x^7 - 210*x^6 - 252*x^5 + 126*x^4 + 84*x^3 - 28*x^2 - 8*x + 1
sage: pol48 = x^6 - 4*x^4 + 12*x^2 - 12
sage: pol48.splitting_field('a')
Number Field in a with defining polynomial x^48 ...
If you somehow know the degree of the field in advance, you
should add a ``degree_multiple`` argument. This can speed up the
computation, in particular for polynomials of degree >= 12 or
for relative extensions::
sage: pol15.splitting_field('a', degree_multiple=15)
Number Field in a with defining polynomial x^15 + x^14 - 14*x^13 - 13*x^12 + 78*x^11 + 66*x^10 - 220*x^9 - 165*x^8 + 330*x^7 + 210*x^6 - 252*x^5 - 126*x^4 + 84*x^3 + 28*x^2 - 8*x - 1
A value for ``degree_multiple`` which isn't actually a
multiple of the absolute degree of the splitting field can
either result in a wrong answer or the following exception::
sage: pol48.splitting_field('a', degree_multiple=20)
Traceback (most recent call last):
...
ValueError: inconsistent degree_multiple in splitting_field()
Compute the Galois closure as the splitting field of the defining polynomial::
sage: R.<x> = PolynomialRing(QQ)
sage: pol48 = x^6 - 4*x^4 + 12*x^2 - 12
sage: K.<a> = NumberField(pol48)
sage: L.<b> = pol48.change_ring(K).splitting_field()
sage: L
Number Field in b with defining polynomial x^48 ...
Try all Galois groups over `\QQ` in degree 5 except for `S_5`
(the latter is infeasible with the current implementation)::
sage: C5pol = x^5 + x^4 - 4*x^3 - 3*x^2 + 3*x + 1
sage: C5pol.splitting_field('x')
Number Field in x with defining polynomial x^5 + x^4 - 4*x^3 - 3*x^2 + 3*x + 1
sage: D10pol = x^5 - x^4 - 5*x^3 + 4*x^2 + 3*x - 1
sage: D10pol.splitting_field('x')
Number Field in x with defining polynomial x^10 - 28*x^8 + 216*x^6 - 681*x^4 + 902*x^2 - 401
sage: AGL_1_5pol = x^5 - 2
sage: AGL_1_5pol.splitting_field('x')
Number Field in x with defining polynomial x^20 + 10*x^19 + 55*x^18 + 210*x^17 + 595*x^16 + 1300*x^15 + 2250*x^14 + 3130*x^13 + 3585*x^12 + 3500*x^11 + 2965*x^10 + 2250*x^9 + 1625*x^8 + 1150*x^7 + 750*x^6 + 400*x^5 + 275*x^4 + 100*x^3 + 75*x^2 + 25
sage: A5pol = x^5 - x^4 + 2*x^2 - 2*x + 2
sage: A5pol.splitting_field('x')
Number Field in x with defining polynomial x^60 ...
We can use the ``abort_degree`` option if we don't want to compute
fields of too large degree (this can be used to check whether the
splitting field has small degree)::
sage: (x^5+x+3).splitting_field('b', abort_degree=119)
Traceback (most recent call last):
...
SplittingFieldAbort: degree of splitting field equals 120
sage: (x^10+x+3).splitting_field('b', abort_degree=60) # long time (10s on sage.math, 2014)
Traceback (most recent call last):
...
SplittingFieldAbort: degree of splitting field is a multiple of 180
Use the ``degree_divisor`` attribute to recover the divisor of the
degree of the splitting field or ``degree_multiple`` to recover a
multiple::
sage: from sage.rings.number_field.splitting_field import SplittingFieldAbort
sage: try: # long time (4s on sage.math, 2014)
....: (x^8+x+1).splitting_field('b', abort_degree=60, simplify=False)
....: except SplittingFieldAbort as e:
....: print(e.degree_divisor)
....: print(e.degree_multiple)
120
1440
TESTS::
sage: from sage.rings.number_field.splitting_field import splitting_field
sage: splitting_field(polygen(QQ), name='x', map=True, simplify_all=True)
(Number Field in x with defining polynomial x, Ring morphism:
From: Rational Field
To: Number Field in x with defining polynomial x
Defn: 1 |--> 1)
"""
from sage.misc.all import cputime
from sage.misc.verbose import verbose
degree_multiple = Integer(degree_multiple or 0)
abort_degree = Integer(abort_degree or 0)
# Kpol = PARI polynomial in y defining the extension found so far
F = poly.base_ring()
if is_RationalField(F):
Kpol = pari("'y")
else:
Kpol = F.pari_polynomial("y")
# Fgen = the generator of F as element of Q[y]/Kpol
# (only needed if map=True)
if map:
Fgen = F.gen().__pari__()
verbose("Starting field: %s" % Kpol)
# L and Lred are lists of SplittingData.
# L contains polynomials which are irreducible over K,
# Lred contains polynomials which need to be factored.
L = []
Lred = [SplittingData(poly._pari_with_name(), degree_multiple)]
# Main loop, handle polynomials one by one
while True:
# Absolute degree of current field K
absolute_degree = Integer(Kpol.poldegree())
# Compute minimum relative degree of splitting field
rel_degree_divisor = Integer(1)
for splitting in L:
rel_degree_divisor = rel_degree_divisor.lcm(splitting.poldegree())
# Check for early aborts
abort_rel_degree = abort_degree // absolute_degree
if abort_rel_degree and rel_degree_divisor > abort_rel_degree:
raise SplittingFieldAbort(absolute_degree * rel_degree_divisor,
degree_multiple)
# First, factor polynomials in Lred and store the result in L
verbose("SplittingData to factor: %s" %
[s._repr_tuple() for s in Lred])
t = cputime()
for splitting in Lred:
m = splitting.dm.gcd(degree_multiple).gcd(
factorial(splitting.poldegree()))
if m == 1:
continue
factors = Kpol.nffactor(splitting.pol)[0]
for q in factors:
d = q.poldegree()
fac = factorial(d)
# Multiple of the degree of the splitting field of q,
# note that the degree equals fac iff the Galois group is S_n.
mq = m.gcd(fac)
if mq == 1:
continue
# Multiple of the degree of the splitting field of q
# over the field defined by adding square root of the
# discriminant.
# If the Galois group is contained in A_n, then mq_alt is
# also the degree multiple over the current field K.
# Here, we have equality if the Galois group is A_n.
mq_alt = mq.gcd(fac // 2)
# If we are over Q, then use PARI's polgalois() to compute
# these degrees exactly.
if absolute_degree == 1:
try:
G = q.polgalois()
except PariError:
pass
else:
mq = Integer(G[0])
mq_alt = mq // 2 if (G[1] == -1) else mq
# In degree 4, use the cubic resolvent to refine the
# degree bounds.
if d == 4 and mq >= 12: # mq equals 12 or 24
# Compute cubic resolvent
a0, a1, a2, a3, a4 = (q / q.pollead()).Vecrev()
assert a4 == 1
cubicpol = pari([
4 * a0 * a2 - a1 * a1 - a0 * a3 * a3, a1 * a3 - 4 * a0,
-a2, 1
]).Polrev()
cubicfactors = Kpol.nffactor(cubicpol)[0]
if len(cubicfactors) == 1: # A4 or S4
# After adding a root of the cubic resolvent,
# the degree of the extension defined by q
# is a factor 3 smaller.
L.append(SplittingData(cubicpol, 3))
rel_degree_divisor = rel_degree_divisor.lcm(3)
mq = mq // 3 # 4 or 8
mq_alt = 4
elif len(cubicfactors) == 2: # C4 or D8
# The irreducible degree 2 factor is
# equivalent to x^2 - q.poldisc().
discpol = cubicfactors[1]
L.append(SplittingData(discpol, 2))
mq = mq_alt = 4
else: # C2 x C2
mq = mq_alt = 4
if mq > mq_alt >= 3:
# Add quadratic resolvent x^2 - D to decrease
# the degree multiple by a factor 2.
discpol = pari([-q.poldisc(), 0, 1]).Polrev()
discfactors = Kpol.nffactor(discpol)[0]
if len(discfactors) == 1:
# Discriminant is not a square
L.append(SplittingData(discpol, 2))
rel_degree_divisor = rel_degree_divisor.lcm(2)
mq = mq_alt
L.append(SplittingData(q, mq))
rel_degree_divisor = rel_degree_divisor.lcm(q.poldegree())
if abort_rel_degree and rel_degree_divisor > abort_rel_degree:
raise SplittingFieldAbort(
absolute_degree * rel_degree_divisor, degree_multiple)
verbose("Done factoring", t, level=2)
if len(L) == 0: # Nothing left to do
break
# Recompute absolute degree multiple
new_degree_multiple = absolute_degree
for splitting in L:
new_degree_multiple *= splitting.dm
degree_multiple = new_degree_multiple.gcd(degree_multiple)
# Absolute degree divisor
degree_divisor = rel_degree_divisor * absolute_degree
# Sort according to degree to handle low degrees first
L.sort(key=lambda x: x.key())
verbose("SplittingData to handle: %s" % [s._repr_tuple() for s in L])
verbose("Bounds for absolute degree: [%s, %s]" %
(degree_divisor, degree_multiple))
# Check consistency
if degree_multiple % degree_divisor != 0:
raise ValueError(
"inconsistent degree_multiple in splitting_field()")
for splitting in L:
# The degree of the splitting field must be a multiple of
# the degree of the polynomial. Only do this check for
# SplittingData with minimal dm, because the higher dm are
# defined as relative degree over the splitting field of
# the polynomials with lesser dm.
if splitting.dm > L[0].dm:
break
if splitting.dm % splitting.poldegree() != 0:
raise ValueError(
"inconsistent degree_multiple in splitting_field()")
# Add a root of f = L[0] to construct the field N = K[x]/f(x)
splitting = L[0]
f = splitting.pol
verbose("Handling polynomial %s" % (f.lift()), level=2)
t = cputime()
Npol, KtoN, k = Kpol.rnfequation(f, flag=1)
# Make Npol monic integral primitive, store in Mpol
# (after this, we don't need Npol anymore, only Mpol)
Mdiv = pari(1)
Mpol = Npol
while True:
denom = Integer(Mpol.pollead())
if denom == 1:
break
denom = pari(denom.factor().radical_value())
Mpol = (Mpol * (denom**Mpol.poldegree())).subst(
"x",
pari([0, 1 / denom]).Polrev("x"))
Mpol /= Mpol.content()
Mdiv *= denom
# We are finished for sure if we hit the degree bound
finished = (Mpol.poldegree() >= degree_multiple)
if simplify_all or (simplify and not finished):
# Find a simpler defining polynomial Lpol for Mpol
verbose("New field before simplifying: %s" % Mpol, t)
t = cputime()
M = Mpol.polred(flag=3)
n = len(M[0]) - 1
Lpol = M[1][n].change_variable_name("y")
LtoM = M[0][n].change_variable_name("y").Mod(
Mpol.change_variable_name("y"))
MtoL = LtoM.modreverse()
else:
# Lpol = Mpol
Lpol = Mpol.change_variable_name("y")
MtoL = pari("'y")
NtoL = MtoL / Mdiv
KtoL = KtoN.lift().subst("x", NtoL).Mod(Lpol)
Kpol = Lpol # New Kpol (for next iteration)
verbose("New field: %s" % Kpol, t)
if map:
t = cputime()
Fgen = Fgen.lift().subst("y", KtoL)
verbose("Computed generator of F in K", t, level=2)
if finished:
break
t = cputime()
# Convert f and elements of L from K to L and store in L
# (if the polynomial is certain to remain irreducible) or Lred.
Lold = L[1:]
L = []
Lred = []
# First add f divided by the linear factor we obtained,
# mg is the new degree multiple.
mg = splitting.dm // f.poldegree()
if mg > 1:
g = [c.subst("y", KtoL).Mod(Lpol) for c in f.Vecrev().lift()]
g = pari(g).Polrev()
g /= pari([k * KtoL - NtoL, 1]).Polrev() # divide linear factor
Lred.append(SplittingData(g, mg))
for splitting in Lold:
g = [c.subst("y", KtoL) for c in splitting.pol.Vecrev().lift()]
g = pari(g).Polrev()
mg = splitting.dm
if Integer(g.poldegree()).gcd(
f.poldegree()) == 1: # linearly disjoint fields
L.append(SplittingData(g, mg))
else:
Lred.append(SplittingData(g, mg))
verbose("Converted polynomials to new field", t, level=2)
# Convert Kpol to Sage and construct the absolute number field
Kpol = PolynomialRing(RationalField(),
name=poly.variable_name())(Kpol / Kpol.pollead())
K = NumberField(Kpol, name)
if map:
return K, F.hom(Fgen, K)
else:
return K
def K1_func(SUK, v, A, prec=106):
r"""
Return the constant `K_1` from Smart's TCDF paper, [Sma1995]_
INPUT:
- ``SUK`` -- a group of `S`-units
- ``v`` -- an infinite place of ``K`` (element of ``SUK.number_field().places(prec)``)
- ``A`` -- a list of all products of each potential ``a``, ``b`` in the $S$-unit equation ``ax + by + 1 = 0`` with each root of unity of ``K``
- ``prec`` -- (default: 106) the precision of the real field
OUTPUT:
The constant ``K1,`` a real number
EXAMPLES::
sage: from sage.rings.number_field.S_unit_solver import K1_func
sage: K.<xi> = NumberField(x^3-3)
sage: SUK = UnitGroup(K, S=tuple(K.primes_above(3)))
sage: phi_real = K.places()[0]
sage: phi_complex = K.places()[1]
sage: A = K.roots_of_unity()
sage: K1_func(SUK, phi_real, A)
4.396386097852707394927181864635e16
sage: K1_func(SUK, phi_complex, A)
2.034870098399844430207420286581e17
REFERENCES:
- [Sma1995]_ p. 825
"""
R = RealField(prec)
#[Sma1995]_ p. 825
if is_real_place(v):
c11 = R(4*c4_func(SUK, v, A, prec)).log() / c3_func(SUK, prec)
else:
c11 = 2*( R(4*(c4_func(SUK,v, A, prec)).sqrt()).log() ) / c3_func(SUK, prec)
#[Sma1995]_ p. 825
if is_real_place(v):
c12 = R(2 * c4_func(SUK, v, A, prec))
else:
c12 = R(2 * c4_func(SUK, v, A, prec).sqrt())
#[Sma1998]_ p. 225, Theorem A.1
d = SUK.number_field().degree()
t = SUK.rank()
Baker_C = R(18 * factorial(t+2) * (t+1)**(t+2) * (32*d)**(t+3) * R(2*(t+1) * d).log())
def hprime(SUK, alpha, v):
#[Sma1998]_ p. 225
return R(max(alpha.global_height(), 1/SUK.number_field().degree(), v(alpha).log().abs()/SUK.number_field().degree()))
#[Sma1995]_ p. 825 and [Sma1998]_ p. 225, Theorem A.1
c14 = Baker_C * prod([hprime(SUK, alpha, v) for alpha in SUK.gens_values()])
#[Sma1995]_ p. 825
c15 = 2 * (c12.log()+c14*((SUK.rank()+1)*c14/c13_func(SUK, v, prec)).log()) / c13_func(SUK, v, prec)
return max([c11, c15])
def _closed_form(hyp):
a, b, z = hyp.operands()
a, b = a.operands(), b.operands()
p, q = len(a), len(b)
if z == 0:
return Integer(1)
if p == q == 0:
return exp(z)
if p == 1 and q == 0:
return (1 - z)**(-a[0])
if p == 0 and q == 1:
# TODO: make this require only linear time
def _0f1(b, z):
F12 = cosh(2 * sqrt(z))
F32 = sinh(2 * sqrt(z)) / (2 * sqrt(z))
if 2 * b == 1:
return F12
if 2 * b == 3:
return F32
if 2 * b > 3:
return ((b - 2) * (b - 1) / z *
(_0f1(b - 2, z) - _0f1(b - 1, z)))
if 2 * b < 1:
return (_0f1(b + 1, z) + z / (b *
(b + 1)) * _0f1(b + 2, z))
raise ValueError
# Can evaluate 0F1 in terms of elementary functions when
# the parameter is a half-integer
if 2 * b[0] in ZZ and b[0] not in ZZ:
return _0f1(b[0], z)
# Confluent hypergeometric function
if p == 1 and q == 1:
aa, bb = a[0], b[0]
if aa * 2 == 1 and bb * 2 == 3:
t = sqrt(-z)
return sqrt(pi) / 2 * erf(t) / t
if a == 1 and b == 2:
return (exp(z) - 1) / z
n, m = aa, bb
if n in ZZ and m in ZZ and m > 0 and n > 0:
rf = rising_factorial
if m <= n:
return (exp(z) * sum(
rf(m - n, k) * (-z)**k / factorial(k) / rf(m, k)
for k in range(n - m + 1)))
else:
T = sum(
rf(n - m + 1, k) * z**k / (factorial(k) * rf(2 - m, k))
for k in range(m - n))
U = sum(
rf(1 - n, k) * (-z)**k / (factorial(k) * rf(2 - m, k))
for k in range(n))
return (factorial(m - 2) * rf(1 - m, n) * z**(1 - m) /
factorial(n - 1) * (T - exp(z) * U))
if p == 2 and q == 1:
R12 = QQ((1, 2))
R32 = QQ((3, 2))
def _2f1(a, b, c, z):
"""
Evaluation of 2F1(a, b; c; z), assuming a, b, c positive
integers or half-integers
"""
if b == c:
return (1 - z)**(-a)
if a == c:
return (1 - z)**(-b)
if a == 0 or b == 0:
return Integer(1)
if a > b:
a, b = b, a
if b >= 2:
F1 = _2f1(a, b - 1, c, z)
F2 = _2f1(a, b - 2, c, z)
q = (b - 1) * (z - 1)
return (((c - 2 * b + 2 + (b - a - 1) * z) * F1 +
(b - c - 1) * F2) / q)
if c > 2:
# how to handle this case?
if a - c + 1 == 0 or b - c + 1 == 0:
raise NotImplementedError
F1 = _2f1(a, b, c - 1, z)
F2 = _2f1(a, b, c - 2, z)
r1 = (c - 1) * (2 - c - (a + b - 2 * c + 3) * z)
r2 = (c - 1) * (c - 2) * (1 - z)
q = (a - c + 1) * (b - c + 1) * z
return (r1 * F1 + r2 * F2) / q
if (a, b, c) == (R12, 1, 2):
return (2 - 2 * sqrt(1 - z)) / z
if (a, b, c) == (1, 1, 2):
return -log(1 - z) / z
if (a, b, c) == (1, R32, R12):
return (1 + z) / (1 - z)**2
if (a, b, c) == (1, R32, 2):
return 2 * (1 / sqrt(1 - z) - 1) / z
if (a, b, c) == (R32, 2, R12):
return (1 + 3 * z) / (1 - z)**3
if (a, b, c) == (R32, 2, 1):
return (2 + z) / (2 * (sqrt(1 - z) * (1 - z)**2))
if (a, b, c) == (2, 2, 1):
return (1 + z) / (1 - z)**3
raise NotImplementedError
aa, bb = a
cc, = b
if z == 1:
return (gamma(cc) * gamma(cc - aa - bb) / gamma(cc - aa) /
gamma(cc - bb))
if ((aa * 2) in ZZ and (bb * 2) in ZZ and (cc * 2) in ZZ and aa > 0
and bb > 0 and cc > 0):
try:
return _2f1(aa, bb, cc, z)
except NotImplementedError:
pass
return hyp
def gamma__exact(n):
"""
Evaluates the exact value of the `\Gamma` function at an integer or
half-integer argument.
EXAMPLES::
sage: gamma__exact(4)
6
sage: gamma__exact(3)
2
sage: gamma__exact(2)
1
sage: gamma__exact(1)
1
sage: gamma__exact(1/2)
sqrt(pi)
sage: gamma__exact(3/2)
1/2*sqrt(pi)
sage: gamma__exact(5/2)
3/4*sqrt(pi)
sage: gamma__exact(7/2)
15/8*sqrt(pi)
sage: gamma__exact(-1/2)
-2*sqrt(pi)
sage: gamma__exact(-3/2)
4/3*sqrt(pi)
sage: gamma__exact(-5/2)
-8/15*sqrt(pi)
sage: gamma__exact(-7/2)
16/105*sqrt(pi)
TESTS::
sage: gamma__exact(1/3)
Traceback (most recent call last):
...
TypeError: you must give an integer or half-integer argument
"""
from sage.all import sqrt
n = QQ(n)
if denominator(n) == 1:
if n <= 0:
return infinity
if n > 0:
return factorial(n-1)
elif denominator(n) == 2:
ans = QQ.one()
while n != QQ((1, 2)):
if n < 0:
ans /= n
n += 1
elif n > 0:
n += -1
ans *= n
ans *= sqrt(pi)
return ans
else:
raise TypeError("you must give an integer or half-integer argument")
def zeta__exact(n):
r"""
Returns the exact value of the Riemann Zeta function
The argument must be a critical value, namely either positive even
or negative odd.
See for example [Iwa1972]_, p13, Special value of `\zeta(2k)`
EXAMPLES:
Let us test the accuracy for negative special values::
sage: RR = RealField(100)
sage: for i in range(1,10):
....: print("zeta({}): {}".format(1-2*i, RR(zeta__exact(1-2*i)) - zeta(RR(1-2*i))))
zeta(-1): 0.00000000000000000000000000000
zeta(-3): 0.00000000000000000000000000000
zeta(-5): 0.00000000000000000000000000000
zeta(-7): 0.00000000000000000000000000000
zeta(-9): 0.00000000000000000000000000000
zeta(-11): 0.00000000000000000000000000000
zeta(-13): 0.00000000000000000000000000000
zeta(-15): 0.00000000000000000000000000000
zeta(-17): 0.00000000000000000000000000000
Let us test the accuracy for positive special values::
sage: all(abs(RR(zeta__exact(2*i))-zeta(RR(2*i))) < 10**(-28) for i in range(1,10))
True
TESTS::
sage: zeta__exact(4)
1/90*pi^4
sage: zeta__exact(-3)
1/120
sage: zeta__exact(0)
-1/2
sage: zeta__exact(5)
Traceback (most recent call last):
...
TypeError: n must be a critical value (i.e. even > 0 or odd < 0)
REFERENCES:
- [Iwa1972]_
- [IR1990]_
- [Was1997]_
"""
if n < 0:
return bernoulli(1-n)/(n-1)
elif n > 1:
if (n % 2 == 0):
return ZZ(-1)**(n//2 + 1) * ZZ(2)**(n-1) * pi**n * bernoulli(n) / factorial(n)
else:
raise TypeError("n must be a critical value (i.e. even > 0 or odd < 0)")
elif n == 1:
return infinity
elif n == 0:
return QQ((-1, 2))
def zeta__exact(n):
r"""
Returns the exact value of the Riemann Zeta function
The argument must be a critical value, namely either positive even
or negative odd.
See for example [Iwa1972]_, p13, Special value of `\zeta(2k)`
EXAMPLES:
Let us test the accuracy for negative special values::
sage: RR = RealField(100)
sage: for i in range(1,10):
....: print("zeta({}): {}".format(1-2*i, RR(zeta__exact(1-2*i)) - zeta(RR(1-2*i))))
zeta(-1): 0.00000000000000000000000000000
zeta(-3): 0.00000000000000000000000000000
zeta(-5): 0.00000000000000000000000000000
zeta(-7): 0.00000000000000000000000000000
zeta(-9): 0.00000000000000000000000000000
zeta(-11): 0.00000000000000000000000000000
zeta(-13): 0.00000000000000000000000000000
zeta(-15): 0.00000000000000000000000000000
zeta(-17): 0.00000000000000000000000000000
Let us test the accuracy for positive special values::
sage: all(abs(RR(zeta__exact(2*i))-zeta(RR(2*i))) < 10**(-28) for i in range(1,10))
True
TESTS::
sage: zeta__exact(4)
1/90*pi^4
sage: zeta__exact(-3)
1/120
sage: zeta__exact(0)
-1/2
sage: zeta__exact(5)
Traceback (most recent call last):
...
TypeError: n must be a critical value (i.e. even > 0 or odd < 0)
REFERENCES:
- [Iwa1972]_
- [IR1990]_
- [Was1997]_
"""
if n < 0:
return bernoulli(1 - n) / (n - 1)
elif n > 1:
if (n % 2 == 0):
return ZZ(-1)**(n // 2 + 1) * ZZ(2)**(
n - 1) * pi**n * bernoulli(n) / factorial(n)
else:
raise TypeError(
"n must be a critical value (i.e. even > 0 or odd < 0)")
elif n == 1:
return infinity
elif n == 0:
return QQ((-1, 2))
def bch_iterator(X=None, Y=None):
r"""
A generator function which returns successive terms of the
Baker-Campbell-Hausdorff formula.
INPUT:
- ``X`` -- (optional) an element of a Lie algebra
- ``Y`` -- (optional) an element of a Lie algebra
The BCH formula is an expression for `\log(\exp(X)\exp(Y))` as a sum of Lie
brackets of ``X`` and ``Y`` with rational coefficients. In arbitrary Lie
algebras, the infinite sum is only guaranteed to converge for ``X`` and
``Y`` close to zero.
If the elements ``X`` and ``Y`` are not given, then the iterator will
return successive terms of the abstract BCH formula, i.e., the BCH formula
for the generators of the free Lie algebra on 2 generators.
If the Lie algebra containing ``X`` and ``Y`` is not nilpotent, the
iterator will output infinitely many elements. If the Lie algebra is
nilpotent, the number of elements outputted is equal to the nilpotency step.
EXAMPLES:
The terms of the abstract BCH formula up to fifth order brackets::
sage: from sage.algebras.lie_algebras.bch import bch_iterator
sage: bch = bch_iterator()
sage: next(bch)
X + Y
sage: next(bch)
1/2*[X, Y]
sage: next(bch)
1/12*[X, [X, Y]] + 1/12*[[X, Y], Y]
sage: next(bch)
1/24*[X, [[X, Y], Y]]
sage: next(bch)
-1/720*[X, [X, [X, [X, Y]]]] + 1/180*[X, [X, [[X, Y], Y]]]
+ 1/360*[[X, [X, Y]], [X, Y]] + 1/180*[X, [[[X, Y], Y], Y]]
+ 1/120*[[X, Y], [[X, Y], Y]] - 1/720*[[[[X, Y], Y], Y], Y]
For nilpotent Lie algebras the BCH formula only has finitely many terms::
sage: L = LieAlgebra(QQ, 2, step=3)
sage: L.inject_variables()
Defining X_1, X_2, X_12, X_112, X_122
sage: [Z for Z in bch_iterator(X_1, X_2)]
[X_1 + X_2, 1/2*X_12, 1/12*X_112 + 1/12*X_122]
sage: [Z for Z in bch_iterator(X_1 + X_2, X_12)]
[X_1 + X_2 + X_12, 1/2*X_112 - 1/2*X_122, 0]
The elements ``X`` and ``Y`` don't need to be elements of the same Lie
algebra if there is a coercion from one to the other::
sage: L = LieAlgebra(QQ, 3, step=2)
sage: L.inject_variables()
Defining X_1, X_2, X_3, X_12, X_13, X_23
sage: S = L.subalgebra(X_1, X_2)
sage: bch1 = [Z for Z in bch_iterator(S(X_1), S(X_2))]; bch1
[X_1 + X_2, 1/2*X_12]
sage: bch1[0].parent() == S
True
sage: bch2 = [Z for Z in bch_iterator(S(X_1), X_3)]; bch2
[X_1 + X_3, 1/2*X_13]
sage: bch2[0].parent() == L
True
The BCH formula requires a coercion from the rationals::
sage: L.<X,Y,Z> = LieAlgebra(ZZ, 2, step=2)
sage: bch = bch_iterator(X, Y); next(bch)
Traceback (most recent call last):
...
TypeError: the BCH formula is not well defined since Integer Ring has no coercion from Rational Field
TESTS:
Compare to the BCH formula up to degree 5 given by wikipedia::
sage: from sage.algebras.lie_algebras.bch import bch_iterator
sage: bch = bch_iterator()
sage: L.<X,Y> = LieAlgebra(QQ)
sage: L = L.Lyndon()
sage: computed_BCH = L.sum(next(bch) for k in range(5))
sage: wikiBCH = X + Y + 1/2*L[X,Y] + 1/12*(L[X,[X,Y]] + L[Y,[Y,X]])
sage: wikiBCH += -1/24*L[Y,[X,[X,Y]]]
sage: wikiBCH += -1/720*(L[Y,[Y,[Y,[Y,X]]]] + L[X,[X,[X,[X,Y]]]])
sage: wikiBCH += 1/360*(L[X,[Y,[Y,[Y,X]]]] + L[Y,[X,[X,[X,Y]]]])
sage: wikiBCH += 1/120*(L[Y,[X,[Y,[X,Y]]]] + L[X,[Y,[X,[Y,X]]]])
sage: computed_BCH == wikiBCH
True
ALGORITHM:
The BCH formula `\log(\exp(X)\exp(Y)) = \sum_k Z_k` is computed starting
from `Z_1 = X + Y`, by the recursion
.. MATH::
(m+1)Z_{m+1} = \frac{1}{2}[X - Y, Z_m]
+ \sum_{2\leq 2p \leq m}\frac{B_{2p}}{(2p)!}\sum_{k_1+\cdots+k_{2p}=m}
[Z_{k_1}, [\cdots [Z_{k_{2p}}, X + Y]\cdots],
where `B_{2p}` are the Bernoulli numbers, see Lemma 2.15.3. in [Var1984]_.
.. WARNING::
The time needed to compute each successive term increases exponentially.
For example on one machine iterating through `Z_{11},...,Z_{18}` for a
free Lie algebra, computing each successive term took 4-5 times longer,
going from 0.1s for `Z_{11}` to 21 minutes for `Z_{18}`.
"""
if X is None or Y is None:
L = LieAlgebra(QQ, ['X', 'Y']).Lyndon()
X, Y = L.lie_algebra_generators()
else:
X, Y = canonical_coercion(X, Y)
L = X.parent()
R = L.base_ring()
if not R.has_coerce_map_from(QQ):
raise TypeError("the BCH formula is not well defined since %s "
"has no coercion from %s" % (R, QQ))
xdif = X - Y
Z = [0, X + Y] # 1-based indexing for convenience
m = 1
yield Z[1]
while True:
m += 1
if L in LieAlgebras.Nilpotent and m > L.step():
return
# apply the recursion formula of [Var1984]
Zm = ~QQ(2 * m) * xdif.bracket(Z[-1])
for p in range(1, (m - 1) // 2 + 1):
partitions = IntegerListsLex(m - 1, length=2 * p, min_part=1)
coeff = bernoulli(2 * p) / QQ(m * factorial(2 * p))
for kvec in partitions:
W = Z[1]
for k in kvec:
W = Z[k].bracket(W)
Zm += coeff * W
Z.append(Zm)
yield Zm