"""Groebner bases algorithms. """
from __future__ import print_function, division
from sympy.core.compatibility import range
from sympy.core.symbol import Dummy
from sympy.polys.monomials import monomial_mul, monomial_lcm, monomial_divides, term_div
from sympy.polys.orderings import lex
from sympy.polys.polyerrors import DomainError
from sympy.polys.polyconfig import query
[docs]def groebner(seq, ring, method=None):
"""
Computes Groebner basis for a set of polynomials in `K[X]`.
Wrapper around the (default) improved Buchberger and the other algorithms
for computing Groebner bases. The choice of algorithm can be changed via
``method`` argument or :func:`setup` from :mod:`sympy.polys.polyconfig`,
where ``method`` can be either ``buchberger`` or ``f5b``.
"""
if method is None:
method = query('groebner')
_groebner_methods = {
'buchberger': _buchberger,
'f5b': _f5b,
}
try:
_groebner = _groebner_methods[method]
except KeyError:
raise ValueError("'%s' is not a valid Groebner bases algorithm (valid are 'buchberger' and 'f5b')" % method)
domain, orig = ring.domain, None
if not domain.is_Field or not domain.has_assoc_Field:
try:
orig, ring = ring, ring.clone(domain=domain.get_field())
except DomainError:
raise DomainError("can't compute a Groebner basis over %s" % domain)
else:
seq = [ s.set_ring(ring) for s in seq ]
G = _groebner(seq, ring)
if orig is not None:
G = [ g.clear_denoms()[1].set_ring(orig) for g in G ]
return G
def _buchberger(f, ring):
"""
Computes Groebner basis for a set of polynomials in `K[X]`.
Given a set of multivariate polynomials `F`, finds another
set `G`, such that Ideal `F = Ideal G` and `G` is a reduced
Groebner basis.
The resulting basis is unique and has monic generators if the
ground domains is a field. Otherwise the result is non-unique
but Groebner bases over e.g. integers can be computed (if the
input polynomials are monic).
Groebner bases can be used to choose specific generators for a
polynomial ideal. Because these bases are unique you can check
for ideal equality by comparing the Groebner bases. To see if
one polynomial lies in an ideal, divide by the elements in the
base and see if the remainder vanishes.
They can also be used to solve systems of polynomial equations
as, by choosing lexicographic ordering, you can eliminate one
variable at a time, provided that the ideal is zero-dimensional
(finite number of solutions).
Notes
=====
Algorithm used: an improved version of Buchberger's algorithm
as presented in T. Becker, V. Weispfenning, Groebner Bases: A
Computational Approach to Commutative Algebra, Springer, 1993,
page 232.
References
==========
.. [1] [Bose03]_
.. [2] [Giovini91]_
.. [3] [Ajwa95]_
.. [4] [Cox97]_
"""
order = ring.order
monomial_mul = ring.monomial_mul
monomial_div = ring.monomial_div
monomial_lcm = ring.monomial_lcm
def select(P):
# normal selection strategy
# select the pair with minimum LCM(LM(f), LM(g))
pr = min(P, key=lambda pair: order(monomial_lcm(f[pair[0]].LM, f[pair[1]].LM)))
return pr
def normal(g, J):
h = g.rem([ f[j] for j in J ])
if not h:
return None
else:
h = h.monic()
if not h in I:
I[h] = len(f)
f.append(h)
return h.LM, I[h]
def update(G, B, ih):
# update G using the set of critical pairs B and h
# [BW] page 230
h = f[ih]
mh = h.LM
# filter new pairs (h, g), g in G
C = G.copy()
D = set()
while C:
# select a pair (h, g) by popping an element from C
ig = C.pop()
g = f[ig]
mg = g.LM
LCMhg = monomial_lcm(mh, mg)
def lcm_divides(ip):
# LCM(LM(h), LM(p)) divides LCM(LM(h), LM(g))
m = monomial_lcm(mh, f[ip].LM)
return monomial_div(LCMhg, m)
# HT(h) and HT(g) disjoint: mh*mg == LCMhg
if monomial_mul(mh, mg) == LCMhg or (
not any(lcm_divides(ipx) for ipx in C) and
not any(lcm_divides(pr[1]) for pr in D)):
D.add((ih, ig))
E = set()
while D:
# select h, g from D (h the same as above)
ih, ig = D.pop()
mg = f[ig].LM
LCMhg = monomial_lcm(mh, mg)
if not monomial_mul(mh, mg) == LCMhg:
E.add((ih, ig))
# filter old pairs
B_new = set()
while B:
# select g1, g2 from B (-> CP)
ig1, ig2 = B.pop()
mg1 = f[ig1].LM
mg2 = f[ig2].LM
LCM12 = monomial_lcm(mg1, mg2)
# if HT(h) does not divide lcm(HT(g1), HT(g2))
if not monomial_div(LCM12, mh) or \
monomial_lcm(mg1, mh) == LCM12 or \
monomial_lcm(mg2, mh) == LCM12:
B_new.add((ig1, ig2))
B_new |= E
# filter polynomials
G_new = set()
while G:
ig = G.pop()
mg = f[ig].LM
if not monomial_div(mg, mh):
G_new.add(ig)
G_new.add(ih)
return G_new, B_new
# end of update ################################
if not f:
return []
# replace f with a reduced list of initial polynomials; see [BW] page 203
f1 = f[:]
while True:
f = f1[:]
f1 = []
for i in range(len(f)):
p = f[i]
r = p.rem(f[:i])
if r:
f1.append(r.monic())
if f == f1:
break
I = {} # ip = I[p]; p = f[ip]
F = set() # set of indices of polynomials
G = set() # set of indices of intermediate would-be Groebner basis
CP = set() # set of pairs of indices of critical pairs
for i, h in enumerate(f):
I[h] = i
F.add(i)
#####################################
# algorithm GROEBNERNEWS2 in [BW] page 232
while F:
# select p with minimum monomial according to the monomial ordering
h = min([f[x] for x in F], key=lambda f: order(f.LM))
ih = I[h]
F.remove(ih)
G, CP = update(G, CP, ih)
# count the number of critical pairs which reduce to zero
reductions_to_zero = 0
while CP:
ig1, ig2 = select(CP)
CP.remove((ig1, ig2))
h = spoly(f[ig1], f[ig2], ring)
# ordering divisors is on average more efficient [Cox] page 111
G1 = sorted(G, key=lambda g: order(f[g].LM))
ht = normal(h, G1)
if ht:
G, CP = update(G, CP, ht[1])
else:
reductions_to_zero += 1
######################################
# now G is a Groebner basis; reduce it
Gr = set()
for ig in G:
ht = normal(f[ig], G - set([ig]))
if ht:
Gr.add(ht[1])
Gr = [f[ig] for ig in Gr]
# order according to the monomial ordering
Gr = sorted(Gr, key=lambda f: order(f.LM), reverse=True)
return Gr
[docs]def spoly(p1, p2, ring):
"""
Compute LCM(LM(p1), LM(p2))/LM(p1)*p1 - LCM(LM(p1), LM(p2))/LM(p2)*p2
This is the S-poly provided p1 and p2 are monic
"""
LM1 = p1.LM
LM2 = p2.LM
LCM12 = ring.monomial_lcm(LM1, LM2)
m1 = ring.monomial_div(LCM12, LM1)
m2 = ring.monomial_div(LCM12, LM2)
s1 = p1.mul_monom(m1)
s2 = p2.mul_monom(m2)
s = s1 - s2
return s
# F5B
# convenience functions
def Sign(f):
return f[0]
def Polyn(f):
return f[1]
def Num(f):
return f[2]
def sig(monomial, index):
return (monomial, index)
def lbp(signature, polynomial, number):
return (signature, polynomial, number)
# signature functions
def sig_cmp(u, v, order):
"""
Compare two signatures by extending the term order to K[X]^n.
u < v iff
- the index of v is greater than the index of u
or
- the index of v is equal to the index of u and u[0] < v[0] w.r.t. order
u > v otherwise
"""
if u[1] > v[1]:
return -1
if u[1] == v[1]:
#if u[0] == v[0]:
# return 0
if order(u[0]) < order(v[0]):
return -1
return 1
def sig_key(s, order):
"""
Key for comparing two signatures.
s = (m, k), t = (n, l)
s < t iff [k > l] or [k == l and m < n]
s > t otherwise
"""
return (-s[1], order(s[0]))
def sig_mult(s, m):
"""
Multiply a signature by a monomial.
The product of a signature (m, i) and a monomial n is defined as
(m * t, i).
"""
return sig(monomial_mul(s[0], m), s[1])
# labeled polynomial functions
def lbp_sub(f, g):
"""
Subtract labeled polynomial g from f.
The signature and number of the difference of f and g are signature
and number of the maximum of f and g, w.r.t. lbp_cmp.
"""
if sig_cmp(Sign(f), Sign(g), Polyn(f).ring.order) < 0:
max_poly = g
else:
max_poly = f
ret = Polyn(f) - Polyn(g)
return lbp(Sign(max_poly), ret, Num(max_poly))
def lbp_mul_term(f, cx):
"""
Multiply a labeled polynomial with a term.
The product of a labeled polynomial (s, p, k) by a monomial is
defined as (m * s, m * p, k).
"""
return lbp(sig_mult(Sign(f), cx[0]), Polyn(f).mul_term(cx), Num(f))
def lbp_cmp(f, g):
"""
Compare two labeled polynomials.
f < g iff
- Sign(f) < Sign(g)
or
- Sign(f) == Sign(g) and Num(f) > Num(g)
f > g otherwise
"""
if sig_cmp(Sign(f), Sign(g), Polyn(f).ring.order) == -1:
return -1
if Sign(f) == Sign(g):
if Num(f) > Num(g):
return -1
#if Num(f) == Num(g):
# return 0
return 1
def lbp_key(f):
"""
Key for comparing two labeled polynomials.
"""
return (sig_key(Sign(f), Polyn(f).ring.order), -Num(f))
# algorithm and helper functions
def critical_pair(f, g, ring):
"""
Compute the critical pair corresponding to two labeled polynomials.
A critical pair is a tuple (um, f, vm, g), where um and vm are
terms such that um * f - vm * g is the S-polynomial of f and g (so,
wlog assume um * f > vm * g).
For performance sake, a critical pair is represented as a tuple
(Sign(um * f), um, f, Sign(vm * g), vm, g), since um * f creates
a new, relatively expensive object in memory, whereas Sign(um *
f) and um are lightweight and f (in the tuple) is a reference to
an already existing object in memory.
"""
domain = ring.domain
ltf = Polyn(f).LT
ltg = Polyn(g).LT
lt = (monomial_lcm(ltf[0], ltg[0]), domain.one)
um = term_div(lt, ltf, domain)
vm = term_div(lt, ltg, domain)
# The full information is not needed (now), so only the product
# with the leading term is considered:
fr = lbp_mul_term(lbp(Sign(f), Polyn(f).leading_term(), Num(f)), um)
gr = lbp_mul_term(lbp(Sign(g), Polyn(g).leading_term(), Num(g)), vm)
# return in proper order, such that the S-polynomial is just
# u_first * f_first - u_second * f_second:
if lbp_cmp(fr, gr) == -1:
return (Sign(gr), vm, g, Sign(fr), um, f)
else:
return (Sign(fr), um, f, Sign(gr), vm, g)
def cp_cmp(c, d):
"""
Compare two critical pairs c and d.
c < d iff
- lbp(c[0], _, Num(c[2]) < lbp(d[0], _, Num(d[2])) (this
corresponds to um_c * f_c and um_d * f_d)
or
- lbp(c[0], _, Num(c[2]) >< lbp(d[0], _, Num(d[2])) and
lbp(c[3], _, Num(c[5])) < lbp(d[3], _, Num(d[5])) (this
corresponds to vm_c * g_c and vm_d * g_d)
c > d otherwise
"""
zero = Polyn(c[2]).ring.zero
c0 = lbp(c[0], zero, Num(c[2]))
d0 = lbp(d[0], zero, Num(d[2]))
r = lbp_cmp(c0, d0)
if r == -1:
return -1
if r == 0:
c1 = lbp(c[3], zero, Num(c[5]))
d1 = lbp(d[3], zero, Num(d[5]))
r = lbp_cmp(c1, d1)
if r == -1:
return -1
#if r == 0:
# return 0
return 1
def cp_key(c, ring):
"""
Key for comparing critical pairs.
"""
return (lbp_key(lbp(c[0], ring.zero, Num(c[2]))), lbp_key(lbp(c[3], ring.zero, Num(c[5]))))
def s_poly(cp):
"""
Compute the S-polynomial of a critical pair.
The S-polynomial of a critical pair cp is cp[1] * cp[2] - cp[4] * cp[5].
"""
return lbp_sub(lbp_mul_term(cp[2], cp[1]), lbp_mul_term(cp[5], cp[4]))
def is_rewritable_or_comparable(sign, num, B):
"""
Check if a labeled polynomial is redundant by checking if its
signature and number imply rewritability or comparability.
(sign, num) is comparable if there exists a labeled polynomial
h in B, such that sign[1] (the index) is less than Sign(h)[1]
and sign[0] is divisible by the leading monomial of h.
(sign, num) is rewritable if there exists a labeled polynomial
h in B, such thatsign[1] is equal to Sign(h)[1], num < Num(h)
and sign[0] is divisible by Sign(h)[0].
"""
for h in B:
# comparable
if sign[1] < Sign(h)[1]:
if monomial_divides(Polyn(h).LM, sign[0]):
return True
# rewritable
if sign[1] == Sign(h)[1]:
if num < Num(h):
if monomial_divides(Sign(h)[0], sign[0]):
return True
return False
def f5_reduce(f, B):
"""
F5-reduce a labeled polynomial f by B.
Continuously searches for non-zero labeled polynomial h in B, such
that the leading term lt_h of h divides the leading term lt_f of
f and Sign(lt_h * h) < Sign(f). If such a labeled polynomial h is
found, f gets replaced by f - lt_f / lt_h * h. If no such h can be
found or f is 0, f is no further F5-reducible and f gets returned.
A polynomial that is reducible in the usual sense need not be
F5-reducible, e.g.:
>>> from sympy.polys.groebnertools import lbp, sig, f5_reduce, Polyn
>>> from sympy.polys import ring, QQ, lex
>>> R, x,y,z = ring("x,y,z", QQ, lex)
>>> f = lbp(sig((1, 1, 1), 4), x, 3)
>>> g = lbp(sig((0, 0, 0), 2), x, 2)
>>> Polyn(f).rem([Polyn(g)])
0
>>> f5_reduce(f, [g])
(((1, 1, 1), 4), x, 3)
"""
order = Polyn(f).ring.order
domain = Polyn(f).ring.domain
if not Polyn(f):
return f
while True:
g = f
for h in B:
if Polyn(h):
if monomial_divides(Polyn(h).LM, Polyn(f).LM):
t = term_div(Polyn(f).LT, Polyn(h).LT, domain)
if sig_cmp(sig_mult(Sign(h), t[0]), Sign(f), order) < 0:
# The following check need not be done and is in general slower than without.
#if not is_rewritable_or_comparable(Sign(gp), Num(gp), B):
hp = lbp_mul_term(h, t)
f = lbp_sub(f, hp)
break
if g == f or not Polyn(f):
return f
def _f5b(F, ring):
"""
Computes a reduced Groebner basis for the ideal generated by F.
f5b is an implementation of the F5B algorithm by Yao Sun and
Dingkang Wang. Similarly to Buchberger's algorithm, the algorithm
proceeds by computing critical pairs, computing the S-polynomial,
reducing it and adjoining the reduced S-polynomial if it is not 0.
Unlike Buchberger's algorithm, each polynomial contains additional
information, namely a signature and a number. The signature
specifies the path of computation (i.e. from which polynomial in
the original basis was it derived and how), the number says when
the polynomial was added to the basis. With this information it
is (often) possible to decide if an S-polynomial will reduce to
0 and can be discarded.
Optimizations include: Reducing the generators before computing
a Groebner basis, removing redundant critical pairs when a new
polynomial enters the basis and sorting the critical pairs and
the current basis.
Once a Groebner basis has been found, it gets reduced.
References
==========
.. [1] Yao Sun, Dingkang Wang: "A New Proof for the Correctness of F5
(F5-Like) Algorithm", http://arxiv.org/abs/1004.0084 (specifically
v4)
.. [2] Thomas Becker, Volker Weispfenning, Groebner bases: A computational
approach to commutative algebra, 1993, p. 203, 216
"""
order = ring.order
# reduce polynomials (like in Mario Pernici's implementation) (Becker, Weispfenning, p. 203)
B = F
while True:
F = B
B = []
for i in range(len(F)):
p = F[i]
r = p.rem(F[:i])
if r:
B.append(r)
if F == B:
break
# basis
B = [lbp(sig(ring.zero_monom, i + 1), F[i], i + 1) for i in range(len(F))]
B.sort(key=lambda f: order(Polyn(f).LM), reverse=True)
# critical pairs
CP = [critical_pair(B[i], B[j], ring) for i in range(len(B)) for j in range(i + 1, len(B))]
CP.sort(key=lambda cp: cp_key(cp, ring), reverse=True)
k = len(B)
reductions_to_zero = 0
while len(CP):
cp = CP.pop()
# discard redundant critical pairs:
if is_rewritable_or_comparable(cp[0], Num(cp[2]), B):
continue
if is_rewritable_or_comparable(cp[3], Num(cp[5]), B):
continue
s = s_poly(cp)
p = f5_reduce(s, B)
p = lbp(Sign(p), Polyn(p).monic(), k + 1)
if Polyn(p):
# remove old critical pairs, that become redundant when adding p:
indices = []
for i, cp in enumerate(CP):
if is_rewritable_or_comparable(cp[0], Num(cp[2]), [p]):
indices.append(i)
elif is_rewritable_or_comparable(cp[3], Num(cp[5]), [p]):
indices.append(i)
for i in reversed(indices):
del CP[i]
# only add new critical pairs that are not made redundant by p:
for g in B:
if Polyn(g):
cp = critical_pair(p, g, ring)
if is_rewritable_or_comparable(cp[0], Num(cp[2]), [p]):
continue
elif is_rewritable_or_comparable(cp[3], Num(cp[5]), [p]):
continue
CP.append(cp)
# sort (other sorting methods/selection strategies were not as successful)
CP.sort(key=lambda cp: cp_key(cp, ring), reverse=True)
# insert p into B:
m = Polyn(p).LM
if order(m) <= order(Polyn(B[-1]).LM):
B.append(p)
else:
for i, q in enumerate(B):
if order(m) > order(Polyn(q).LM):
B.insert(i, p)
break
k += 1
#print(len(B), len(CP), "%d critical pairs removed" % len(indices))
else:
reductions_to_zero += 1
# reduce Groebner basis:
H = [Polyn(g).monic() for g in B]
H = red_groebner(H, ring)
return sorted(H, key=lambda f: order(f.LM), reverse=True)
[docs]def red_groebner(G, ring):
"""
Compute reduced Groebner basis, from BeckerWeispfenning93, p. 216
Selects a subset of generators, that already generate the ideal
and computes a reduced Groebner basis for them.
"""
def reduction(P):
"""
The actual reduction algorithm.
"""
Q = []
for i, p in enumerate(P):
h = p.rem(P[:i] + P[i + 1:])
if h:
Q.append(h)
return [p.monic() for p in Q]
F = G
H = []
while F:
f0 = F.pop()
if not any(monomial_divides(f.LM, f0.LM) for f in F + H):
H.append(f0)
# Becker, Weispfenning, p. 217: H is Groebner basis of the ideal generated by G.
return reduction(H)
[docs]def is_groebner(G, ring):
"""
Check if G is a Groebner basis.
"""
for i in range(len(G)):
for j in range(i + 1, len(G)):
s = spoly(G[i], G[j], ring)
s = s.rem(G)
if s:
return False
return True
[docs]def is_minimal(G, ring):
"""
Checks if G is a minimal Groebner basis.
"""
order = ring.order
domain = ring.domain
G.sort(key=lambda g: order(g.LM))
for i, g in enumerate(G):
if g.LC != domain.one:
return False
for h in G[:i] + G[i + 1:]:
if monomial_divides(h.LM, g.LM):
return False
return True
[docs]def is_reduced(G, ring):
"""
Checks if G is a reduced Groebner basis.
"""
order = ring.order
domain = ring.domain
G.sort(key=lambda g: order(g.LM))
for i, g in enumerate(G):
if g.LC != domain.one:
return False
for term in g:
for h in G[:i] + G[i + 1:]:
if monomial_divides(h.LM, term[0]):
return False
return True
def groebner_lcm(f, g):
"""
Computes LCM of two polynomials using Groebner bases.
The LCM is computed as the unique generator of the intersection
of the two ideals generated by `f` and `g`. The approach is to
compute a Groebner basis with respect to lexicographic ordering
of `t*f` and `(1 - t)*g`, where `t` is an unrelated variable and
then filtering out the solution that doesn't contain `t`.
References
==========
.. [1] [Cox97]_
"""
if f.ring != g.ring:
raise ValueError("Values should be equal")
ring = f.ring
domain = ring.domain
if not f or not g:
return ring.zero
if len(f) <= 1 and len(g) <= 1:
monom = monomial_lcm(f.LM, g.LM)
coeff = domain.lcm(f.LC, g.LC)
return ring.term_new(monom, coeff)
fc, f = f.primitive()
gc, g = g.primitive()
lcm = domain.lcm(fc, gc)
f_terms = [ ((1,) + monom, coeff) for monom, coeff in f.terms() ]
g_terms = [ ((0,) + monom, coeff) for monom, coeff in g.terms() ] \
+ [ ((1,) + monom,-coeff) for monom, coeff in g.terms() ]
t = Dummy("t")
t_ring = ring.clone(symbols=(t,) + ring.symbols, order=lex)
F = t_ring.from_terms(f_terms)
G = t_ring.from_terms(g_terms)
basis = groebner([F, G], t_ring)
def is_independent(h, j):
return all(not monom[j] for monom in h.monoms())
H = [ h for h in basis if is_independent(h, 0) ]
h_terms = [ (monom[1:], coeff*lcm) for monom, coeff in H[0].terms() ]
h = ring.from_terms(h_terms)
return h
def groebner_gcd(f, g):
"""Computes GCD of two polynomials using Groebner bases. """
if f.ring != g.ring:
raise ValueError("Values should be equal")
domain = f.ring.domain
if not domain.is_Field:
fc, f = f.primitive()
gc, g = g.primitive()
gcd = domain.gcd(fc, gc)
H = (f*g).quo([groebner_lcm(f, g)])
if len(H) != 1:
raise ValueError("Length should be 1")
h = H[0]
if not domain.is_Field:
return gcd*h
else:
return h.monic()
