class FpGroup(DefaultPrinting): """ The FpGroup would take a FreeGroup and a list/tuple of relators, the relators would be specified in such a way that each of them be equal to the identity of the provided free group. """ is_group = True is_FpGroup = True is_PermutationGroup = False def __init__(self, fr_grp, relators): relators = _parse_relators(relators) self.free_group = fr_grp self.relators = relators self.generators = self._generators() self.dtype = type("FpGroupElement", (FpGroupElement, ), {"group": self}) # CosetTable instance on identity subgroup self._coset_table = None # returns whether coset table on identity subgroup # has been standardized self._is_standardized = False self._order = None self._center = None self._rewriting_system = RewritingSystem(self) return def _generators(self): return self.free_group.generators def make_confluent(self): ''' Try to make the group's rewriting system confluent ''' self._rewriting_system.make_confluent() return def reduce(self, word): ''' Return the reduced form of `word` in `self` according to the group's rewriting system. If it's confluent, the reduced form is the unique normal form of the word in the group. ''' return self._rewriting_system.reduce(word) def equals(self, word1, word2): ''' Compare `word1` and `word2` for equality in the group using the group's rewriting system. If the system is confluent, the returned answer is necessarily correct. (If it isn't, `False` could be returned in some cases where in fact `word1 == word2`) ''' if self.reduce(word1 * word2**-1) == self.identity: return True elif self._rewriting_system.is_confluent: return False return None @property def identity(self): return self.free_group.identity def __contains__(self, g): return g in self.free_group def subgroup(self, gens, C=None): ''' Return the subgroup generated by `gens` using the Reidemeister-Schreier algorithm ''' if not all([isinstance(g, FreeGroupElement) for g in gens]): raise ValueError("Generators must be `FreeGroupElement`s") if not all([g.group == self.free_group for g in gens]): raise ValueError("Given generators are not members of the group") g, rels = reidemeister_presentation(self, gens, C=C) g = FpGroup(g[0].group, rels) return g def coset_enumeration(self, H, strategy="relator_based", max_cosets=None): """ Return an instance of ``coset table``, when Todd-Coxeter algorithm is run over the ``self`` with ``H`` as subgroup, using ``strategy`` argument as strategy. The returned coset table is compressed but not standardized. """ if not max_cosets: max_cosets = CosetTable.coset_table_max_limit if strategy == 'relator_based': C = coset_enumeration_r(self, H, max_cosets=max_cosets) else: C = coset_enumeration_c(self, H, max_cosets=max_cosets) C.compress() return C def standardize_coset_table(self): """ Standardized the coset table ``self`` and makes the internal variable ``_is_standardized`` equal to ``True``. """ self._coset_table.standardize() self._is_standardized = True def coset_table(self, H, strategy="relator_based"): """ Return the mathematical coset table of ``self`` in ``H``. """ if not H: if self._coset_table != None: if not self._is_standardized: self.standardize_coset_table() else: C = self.coset_enumeration([], strategy) self._coset_table = C self.standardize_coset_table() return self._coset_table.table else: C = self.coset_enumeration(H, strategy) C.standardize() return C.table def order(self, strategy="relator_based"): """ Returns the order of the finitely presented group ``self``. It uses the coset enumeration with identity group as subgroup, i.e ``H=[]``. Examples ======== >>> from sympy.combinatorics.free_groups import free_group >>> from sympy.combinatorics.fp_groups import FpGroup >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x, y**2]) >>> f.order(strategy="coset_table_based") 2 """ from sympy import S, gcd if self._order != None: return self._order if self._coset_table != None: self._order = len(self._coset_table.table) elif len(self.generators) == 1: self._order = gcd([r.array_form[0][1] for r in self.relators]) elif self._is_infinite(): self._order = S.Infinity else: gens, C = self._finite_index_subgroup() if C: ind = len(C.table) self._order = ind * self.subgroup(gens, C=C).order() else: self._order = self.index([]) return self._order def _is_infinite(self): ''' Test if the group is infinite. Return `True` if the test succeeds and `None` otherwise ''' # Abelianisation test: check is the abelianisation is infinite abelian_rels = [] from sympy.polys.solvers import RawMatrix as Matrix from sympy.polys.domains import ZZ from sympy.matrices.normalforms import invariant_factors for rel in self.relators: abelian_rels.append([rel.exponent_sum(g) for g in self.generators]) m = Matrix(abelian_rels) setattr(m, "ring", ZZ) if 0 in invariant_factors(m): return True else: return None def _finite_index_subgroup(self, s=[]): ''' Find the elements of `self` that generate a finite index subgroup and, if found, return the list of elements and the coset table of `self` by the subgroup, otherwise return `(None, None)` ''' gen = self.most_frequent_generator() rels = list(self.generators) rels.extend(self.relators) if not s: if len(self.generators) == 2: s = [gen] + [g for g in self.generators if g != gen] else: rand = self.free_group.identity i = 0 while ((rand in rels or rand**-1 in rels or rand.is_identity) and i < 10): rand = self.random() i += 1 s = [gen, rand] + [g for g in self.generators if g != gen] mid = (len(s) + 1) // 2 half1 = s[:mid] half2 = s[mid:] m = 200 C = None while not C and (m / 2 < CosetTable.coset_table_max_limit): m = min(m, CosetTable.coset_table_max_limit) try: C = self.coset_enumeration(half1, max_cosets=m) half = half1 except ValueError: pass if not C: try: C = self.coset_enumeration(half2, max_cosets=m) half = half2 except ValueError: m *= 2 continue if not C: return None, None C.compress() return half, C def most_frequent_generator(self): gens = self.generators rels = self.relators freqs = [sum([r.generator_count(g) for r in rels]) for g in gens] return gens[freqs.index(max(freqs))] def random(self): import random r = self.free_group.identity for i in range(random.randint(2, 3)): r = r * random.choice(self.generators)**random.choice([1, -1]) return r def index(self, H, strategy="relator_based"): """ Return the index of subgroup ``H`` in group ``self``. Examples ======== >>> from sympy.combinatorics.free_groups import free_group >>> from sympy.combinatorics.fp_groups import FpGroup >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x**5, y**4, y*x*y**3*x**3]) >>> f.index([x]) 4 """ # TODO: use |G:H| = |G|/|H| (currently H can't be made into a group) # when we know |G| and |H| if H == []: return self.order() else: C = self.coset_enumeration(H, strategy) return len(C.table) def __str__(self): if self.free_group.rank > 30: str_form = "<fp group with %s generators>" % self.free_group.rank else: str_form = "<fp group on the generators %s>" % str(self.generators) return str_form __repr__ = __str__
class FpGroup(DefaultPrinting): """ The FpGroup would take a FreeGroup and a list/tuple of relators, the relators would be specified in such a way that each of them be equal to the identity of the provided free group. """ is_group = True is_FpGroup = True is_PermutationGroup = False def __init__(self, fr_grp, relators): relators = _parse_relators(relators) self.free_group = fr_grp self.relators = relators self.generators = self._generators() self.dtype = type("FpGroupElement", (FpGroupElement,), {"group": self}) # CosetTable instance on identity subgroup self._coset_table = None # returns whether coset table on identity subgroup # has been standardized self._is_standardized = False self._order = None self._center = None self._rewriting_system = RewritingSystem(self) return def _generators(self): return self.free_group.generators def make_confluent(self): ''' Try to make the group's rewriting system confluent ''' self._rewriting_system.make_confluent() return def reduce(self, word): ''' Return the reduced form of `word` in `self` according to the group's rewriting system. If it's confluent, the reduced form is the unique normal form of the word in the group. ''' return self._rewriting_system.reduce(word) def equals(self, word1, word2): ''' Compare `word1` and `word2` for equality in the group using the group's rewriting system. If the system is confluent, the returned answer is necessarily correct. (If it isn't, `False` could be returned in some cases where in fact `word1 == word2`) ''' if self.reduce(word1*word2**-1) == self.identity: return True elif self._rewriting_system.is_confluent: return False return None @property def identity(self): return self.free_group.identity def __contains__(self, g): return g in self.free_group def subgroup(self, gens, C=None): ''' Return the subgroup generated by `gens` using the Reidemeister-Schreier algorithm ''' if not all([isinstance(g, FreeGroupElement) for g in gens]): raise ValueError("Generators must be `FreeGroupElement`s") if not all([g.group == self.free_group for g in gens]): raise ValueError("Given generators are not members of the group") g, rels = reidemeister_presentation(self, gens, C=C) g = FpGroup(g[0].group, rels) return g def coset_enumeration(self, H, strategy="relator_based", max_cosets=None): """ Return an instance of ``coset table``, when Todd-Coxeter algorithm is run over the ``self`` with ``H`` as subgroup, using ``strategy`` argument as strategy. The returned coset table is compressed but not standardized. """ if not max_cosets: max_cosets = CosetTable.coset_table_max_limit if strategy == 'relator_based': C = coset_enumeration_r(self, H, max_cosets=max_cosets) else: C = coset_enumeration_c(self, H, max_cosets=max_cosets) C.compress() return C def standardize_coset_table(self): """ Standardized the coset table ``self`` and makes the internal variable ``_is_standardized`` equal to ``True``. """ self._coset_table.standardize() self._is_standardized = True def coset_table(self, H, strategy="relator_based"): """ Return the mathematical coset table of ``self`` in ``H``. """ if not H: if self._coset_table != None: if not self._is_standardized: self.standardize_coset_table() else: C = self.coset_enumeration([], strategy) self._coset_table = C self.standardize_coset_table() return self._coset_table.table else: C = self.coset_enumeration(H, strategy) C.standardize() return C.table def order(self, strategy="relator_based"): """ Returns the order of the finitely presented group ``self``. It uses the coset enumeration with identity group as subgroup, i.e ``H=[]``. Examples ======== >>> from sympy.combinatorics.free_groups import free_group >>> from sympy.combinatorics.fp_groups import FpGroup >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x, y**2]) >>> f.order(strategy="coset_table_based") 2 """ from sympy import S, gcd if self._order != None: return self._order if self._coset_table != None: self._order = len(self._coset_table.table) elif len(self.generators) == 1: self._order = gcd([r.array_form[0][1] for r in self.relators]) elif self._is_infinite(): self._order = S.Infinity else: gens, C = self._finite_index_subgroup() if C: ind = len(C.table) self._order = ind*self.subgroup(gens, C=C).order() else: self._order = self.index([]) return self._order def _is_infinite(self): ''' Test if the group is infinite. Return `True` if the test succeeds and `None` otherwise ''' # Abelianisation test: check is the abelianisation is infinite abelian_rels = [] from sympy.polys.solvers import RawMatrix as Matrix from sympy.polys.domains import ZZ from sympy.matrices.normalforms import invariant_factors for rel in self.relators: abelian_rels.append([rel.exponent_sum(g) for g in self.generators]) m = Matrix(abelian_rels) setattr(m, "ring", ZZ) if 0 in invariant_factors(m): return True else: return None def _finite_index_subgroup(self, s=[]): ''' Find the elements of `self` that generate a finite index subgroup and, if found, return the list of elements and the coset table of `self` by the subgroup, otherwise return `(None, None)` ''' gen = self.most_frequent_generator() rels = list(self.generators) rels.extend(self.relators) if not s: if len(self.generators) == 2: s = [gen] + [g for g in self.generators if g != gen] else: rand = self.free_group.identity i = 0 while ((rand in rels or rand**-1 in rels or rand.is_identity) and i<10): rand = self.random_element() i += 1 s = [gen, rand] + [g for g in self.generators if g != gen] mid = (len(s)+1)//2 half1 = s[:mid] half2 = s[mid:] m = 200 C = None while not C and (m/2 < CosetTable.coset_table_max_limit): m = min(m, CosetTable.coset_table_max_limit) try: C = self.coset_enumeration(half1, max_cosets=m) half = half1 except ValueError: pass if not C: try: C = self.coset_enumeration(half2, max_cosets=m) half = half2 except ValueError: m *= 2 continue if not C: return None, None C.compress() return half, C def most_frequent_generator(self): gens = self.generators rels = self.relators freqs = [sum([r.generator_count(g) for r in rels]) for g in gens] return gens[freqs.index(max(freqs))] def random_element(self): import random r = self.free_group.identity for i in range(random.randint(2,3)): r = r*random.choice(self.generators)**random.choice([1,-1]) return r def index(self, H, strategy="relator_based"): """ Return the index of subgroup ``H`` in group ``self``. Examples ======== >>> from sympy.combinatorics.free_groups import free_group >>> from sympy.combinatorics.fp_groups import FpGroup >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x**5, y**4, y*x*y**3*x**3]) >>> f.index([x]) 4 """ # TODO: use |G:H| = |G|/|H| (currently H can't be made into a group) # when we know |G| and |H| if H == []: return self.order() else: C = self.coset_enumeration(H, strategy) return len(C.table) def __str__(self): if self.free_group.rank > 30: str_form = "<fp group with %s generators>" % self.free_group.rank else: str_form = "<fp group on the generators %s>" % str(self.generators) return str_form __repr__ = __str__
class FpGroup(DefaultPrinting): """ The FpGroup would take a FreeGroup and a list/tuple of relators, the relators would be specified in such a way that each of them be equal to the identity of the provided free group. """ is_group = True is_FpGroup = True is_PermutationGroup = False def __init__(self, fr_grp, relators): relators = _parse_relators(relators) self.free_group = fr_grp self.relators = relators self.generators = self._generators() self.dtype = type("FpGroupElement", (FpGroupElement,), {"group": self}) # CosetTable instance on identity subgroup self._coset_table = None # returns whether coset table on identity subgroup # has been standardized self._is_standardized = False self._order = None self._center = None self._rewriting_system = RewritingSystem(self) self._perm_isomorphism = None return def _generators(self): return self.free_group.generators def make_confluent(self): ''' Try to make the group's rewriting system confluent ''' self._rewriting_system.make_confluent() return def reduce(self, word): ''' Return the reduced form of `word` in `self` according to the group's rewriting system. If it's confluent, the reduced form is the unique normal form of the word in the group. ''' return self._rewriting_system.reduce(word) def equals(self, word1, word2): ''' Compare `word1` and `word2` for equality in the group using the group's rewriting system. If the system is confluent, the returned answer is necessarily correct. (If it isn't, `False` could be returned in some cases where in fact `word1 == word2`) ''' if self.reduce(word1*word2**-1) == self.identity: return True elif self._rewriting_system.is_confluent: return False return None @property def identity(self): return self.free_group.identity def __contains__(self, g): return g in self.free_group def subgroup(self, gens, C=None, homomorphism=False): ''' Return the subgroup generated by `gens` using the Reidemeister-Schreier algorithm homomorphism -- When set to True, return a dictionary containing the images of the presentation generators in the original group. Examples ======== >>> from sympy.combinatorics.fp_groups import (FpGroup, FpSubgroup) >>> from sympy.combinatorics.free_groups import free_group >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x**3, y**5, (x*y)**2]) >>> H = [x*y, x**-1*y**-1*x*y*x] >>> K, T = f.subgroup(H, homomorphism=True) >>> T(K.generators) [x*y, x**-1*y**2*x**-1] ''' if not all([isinstance(g, FreeGroupElement) for g in gens]): raise ValueError("Generators must be `FreeGroupElement`s") if not all([g.group == self.free_group for g in gens]): raise ValueError("Given generators are not members of the group") if homomorphism: g, rels, _gens = reidemeister_presentation(self, gens, C=C, homomorphism=True) else: g, rels = reidemeister_presentation(self, gens, C=C) if g: g = FpGroup(g[0].group, rels) else: g = FpGroup(free_group('')[0], []) if homomorphism: from sympy.combinatorics.homomorphisms import homomorphism return g, homomorphism(g, self, g.generators, _gens, check=False) return g def coset_enumeration(self, H, strategy="relator_based", max_cosets=None, draft=None, incomplete=False): """ Return an instance of ``coset table``, when Todd-Coxeter algorithm is run over the ``self`` with ``H`` as subgroup, using ``strategy`` argument as strategy. The returned coset table is compressed but not standardized. An instance of `CosetTable` for `fp_grp` can be passed as the keyword argument `draft` in which case the coset enumeration will start with that instance and attempt to complete it. When `incomplete` is `True` and the function is unable to complete for some reason, the partially complete table will be returned. """ if not max_cosets: max_cosets = CosetTable.coset_table_max_limit if strategy == 'relator_based': C = coset_enumeration_r(self, H, max_cosets=max_cosets, draft=draft, incomplete=incomplete) else: C = coset_enumeration_c(self, H, max_cosets=max_cosets, draft=draft, incomplete=incomplete) if C.is_complete(): C.compress() return C def standardize_coset_table(self): """ Standardized the coset table ``self`` and makes the internal variable ``_is_standardized`` equal to ``True``. """ self._coset_table.standardize() self._is_standardized = True def coset_table(self, H, strategy="relator_based", max_cosets=None, draft=None, incomplete=False): """ Return the mathematical coset table of ``self`` in ``H``. """ if not H: if self._coset_table is not None: if not self._is_standardized: self.standardize_coset_table() else: C = self.coset_enumeration([], strategy, max_cosets=max_cosets, draft=draft, incomplete=incomplete) self._coset_table = C self.standardize_coset_table() return self._coset_table.table else: C = self.coset_enumeration(H, strategy, max_cosets=max_cosets, draft=draft, incomplete=incomplete) C.standardize() return C.table def order(self, strategy="relator_based"): """ Returns the order of the finitely presented group ``self``. It uses the coset enumeration with identity group as subgroup, i.e ``H=[]``. Examples ======== >>> from sympy.combinatorics.free_groups import free_group >>> from sympy.combinatorics.fp_groups import FpGroup >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x, y**2]) >>> f.order(strategy="coset_table_based") 2 """ from sympy import S, gcd if self._order is not None: return self._order if self._coset_table is not None: self._order = len(self._coset_table.table) elif len(self.relators) == 0: self._order = self.free_group.order() elif len(self.generators) == 1: self._order = abs(gcd([r.array_form[0][1] for r in self.relators])) elif self._is_infinite(): self._order = S.Infinity else: gens, C = self._finite_index_subgroup() if C: ind = len(C.table) self._order = ind*self.subgroup(gens, C=C).order() else: self._order = self.index([]) return self._order def _is_infinite(self): ''' Test if the group is infinite. Return `True` if the test succeeds and `None` otherwise ''' used_gens = set() for r in self.relators: used_gens.update(r.contains_generators()) if any([g not in used_gens for g in self.generators]): return True # Abelianisation test: check is the abelianisation is infinite abelian_rels = [] from sympy.polys.solvers import RawMatrix as Matrix from sympy.polys.domains import ZZ from sympy.matrices.normalforms import invariant_factors for rel in self.relators: abelian_rels.append([rel.exponent_sum(g) for g in self.generators]) m = Matrix(abelian_rels) setattr(m, "ring", ZZ) if 0 in invariant_factors(m): return True else: return None def _finite_index_subgroup(self, s=[]): ''' Find the elements of `self` that generate a finite index subgroup and, if found, return the list of elements and the coset table of `self` by the subgroup, otherwise return `(None, None)` ''' gen = self.most_frequent_generator() rels = list(self.generators) rels.extend(self.relators) if not s: if len(self.generators) == 2: s = [gen] + [g for g in self.generators if g != gen] else: rand = self.free_group.identity i = 0 while ((rand in rels or rand**-1 in rels or rand.is_identity) and i<10): rand = self.random() i += 1 s = [gen, rand] + [g for g in self.generators if g != gen] mid = (len(s)+1)//2 half1 = s[:mid] half2 = s[mid:] draft1 = None draft2 = None m = 200 C = None while not C and (m/2 < CosetTable.coset_table_max_limit): m = min(m, CosetTable.coset_table_max_limit) draft1 = self.coset_enumeration(half1, max_cosets=m, draft=draft1, incomplete=True) if draft1.is_complete(): C = draft1 half = half1 else: draft2 = self.coset_enumeration(half2, max_cosets=m, draft=draft2, incomplete=True) if draft2.is_complete(): C = draft2 half = half2 if not C: m *= 2 if not C: return None, None C.compress() return half, C def most_frequent_generator(self): gens = self.generators rels = self.relators freqs = [sum([r.generator_count(g) for r in rels]) for g in gens] return gens[freqs.index(max(freqs))] def random(self): import random r = self.free_group.identity for i in range(random.randint(2,3)): r = r*random.choice(self.generators)**random.choice([1,-1]) return r def index(self, H, strategy="relator_based"): """ Return the index of subgroup ``H`` in group ``self``. Examples ======== >>> from sympy.combinatorics.free_groups import free_group >>> from sympy.combinatorics.fp_groups import FpGroup >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x**5, y**4, y*x*y**3*x**3]) >>> f.index([x]) 4 """ # TODO: use |G:H| = |G|/|H| (currently H can't be made into a group) # when we know |G| and |H| if H == []: return self.order() else: C = self.coset_enumeration(H, strategy) return len(C.table) def __str__(self): if self.free_group.rank > 30: str_form = "<fp group with %s generators>" % self.free_group.rank else: str_form = "<fp group on the generators %s>" % str(self.generators) return str_form __repr__ = __str__ #============================================================================== # PERMUTATION GROUP METHODS #============================================================================== def _to_perm_group(self): ''' Return an isomorphic permutation group and the isomorphism. The implementation is dependent on coset enumeration so will only terminate for finite groups. ''' from sympy.combinatorics import Permutation, PermutationGroup from sympy.combinatorics.homomorphisms import homomorphism if self.order() is S.Infinity: raise NotImplementedError("Permutation presentation of infinite " "groups is not implemented") if self._perm_isomorphism: T = self._perm_isomorphism P = T.image() else: C = self.coset_table([]) gens = self.generators images = [[C[i][2*gens.index(g)] for i in range(len(C))] for g in gens] images = [Permutation(i) for i in images] P = PermutationGroup(images) T = homomorphism(self, P, gens, images, check=False) self._perm_isomorphism = T return P, T def _perm_group_list(self, method_name, *args): ''' Given the name of a `PermutationGroup` method (returning a subgroup or a list of subgroups) and (optionally) additional arguments it takes, return a list or a list of lists containing the generators of this (or these) subgroups in terms of the generators of `self`. ''' P, T = self._to_perm_group() perm_result = getattr(P, method_name)(*args) single = False if isinstance(perm_result, PermutationGroup): perm_result, single = [perm_result], True result = [] for group in perm_result: gens = group.generators result.append(T.invert(gens)) return result[0] if single else result def derived_series(self): ''' Return the list of lists containing the generators of the subgroups in the derived series of `self`. ''' return self._perm_group_list('derived_series') def lower_central_series(self): ''' Return the list of lists containing the generators of the subgroups in the lower central series of `self`. ''' return self._perm_group_list('lower_central_series') def center(self): ''' Return the list of generators of the center of `self`. ''' return self._perm_group_list('center') def derived_subgroup(self): ''' Return the list of generators of the derived subgroup of `self`. ''' return self._perm_group_list('derived_subgroup') def centralizer(self, other): ''' Return the list of generators of the centralizer of `other` (a list of elements of `self`) in `self`. ''' T = self._to_perm_group()[1] other = T(other) return self._perm_group_list('centralizer', other) def normal_closure(self, other): ''' Return the list of generators of the normal closure of `other` (a list of elements of `self`) in `self`. ''' T = self._to_perm_group()[1] other = T(other) return self._perm_group_list('normal_closure', other) def _perm_property(self, attr): ''' Given an attribute of a `PermutationGroup`, return its value for a permutation group isomorphic to `self`. ''' P = self._to_perm_group()[0] return getattr(P, attr) @property def is_abelian(self): ''' Check if `self` is abelian. ''' return self._perm_property("is_abelian") @property def is_nilpotent(self): ''' Check if `self` is nilpotent. ''' return self._perm_property("is_nilpotent") @property def is_solvable(self): ''' Check if `self` is solvable. ''' return self._perm_property("is_solvable") @property def elements(self): ''' List the elements of `self`. ''' P, T = self._to_perm_group() return T.invert(P._elements) @property def is_cyclic(self): """ Return ``True`` if group is Cyclic. """ if len(self.generators) <= 1: return True try: P, T = self._to_perm_group() except NotImplementedError: raise NotImplementedError("Check for infinite Cyclic group " "is not implemented") return P.is_cyclic def abelian_invariants(self): """ Return Abelian Invariants of a group. """ try: P, T = self._to_perm_group() except NotImplementedError: raise NotImplementedError("abelian invariants is not implemented" "for infinite group") return P.abelian_invariants() def composition_series(self): """ Return subnormal series of maximum length for a group. """ try: P, T = self._to_perm_group() except NotImplementedError: raise NotImplementedError("composition series is not implemented" "for infinite group") return P.composition_series()
class FpGroup(DefaultPrinting): """ The FpGroup would take a FreeGroup and a list/tuple of relators, the relators would be specified in such a way that each of them be equal to the identity of the provided free group. """ is_group = True is_FpGroup = True is_PermutationGroup = False def __init__(self, fr_grp, relators): relators = _parse_relators(relators) self.free_group = fr_grp self.relators = relators self.generators = self._generators() self.dtype = type("FpGroupElement", (FpGroupElement,), {"group": self}) # CosetTable instance on identity subgroup self._coset_table = None # returns whether coset table on identity subgroup # has been standardized self._is_standardized = False self._order = None self._center = None self._rewriting_system = RewritingSystem(self) self._perm_isomorphism = None return def _generators(self): return self.free_group.generators def make_confluent(self): ''' Try to make the group's rewriting system confluent ''' self._rewriting_system.make_confluent() return def reduce(self, word): ''' Return the reduced form of `word` in `self` according to the group's rewriting system. If it's confluent, the reduced form is the unique normal form of the word in the group. ''' return self._rewriting_system.reduce(word) def equals(self, word1, word2): ''' Compare `word1` and `word2` for equality in the group using the group's rewriting system. If the system is confluent, the returned answer is necessarily correct. (If it isn't, `False` could be returned in some cases where in fact `word1 == word2`) ''' if self.reduce(word1*word2**-1) == self.identity: return True elif self._rewriting_system.is_confluent: return False return None @property def identity(self): return self.free_group.identity def __contains__(self, g): return g in self.free_group def subgroup(self, gens, C=None, homomorphism=False): ''' Return the subgroup generated by `gens` using the Reidemeister-Schreier algorithm homomorphism -- When set to True, return a dictionary containing the images of the presentation generators in the original group. Examples ======== >>> from sympy.combinatorics.fp_groups import (FpGroup, FpSubgroup) >>> from sympy.combinatorics.free_groups import free_group >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x**3, y**5, (x*y)**2]) >>> H = [x*y, x**-1*y**-1*x*y*x] >>> K, T = f.subgroup(H, homomorphism=True) >>> T(K.generators) [x*y, x**-1*y**2*x**-1] ''' if not all([isinstance(g, FreeGroupElement) for g in gens]): raise ValueError("Generators must be `FreeGroupElement`s") if not all([g.group == self.free_group for g in gens]): raise ValueError("Given generators are not members of the group") if homomorphism: g, rels, _gens = reidemeister_presentation(self, gens, C=C, homomorphism=True) else: g, rels = reidemeister_presentation(self, gens, C=C) if g: g = FpGroup(g[0].group, rels) else: g = FpGroup(free_group('')[0], []) if homomorphism: from sympy.combinatorics.homomorphisms import homomorphism return g, homomorphism(g, self, g.generators, _gens, check=False) return g def coset_enumeration(self, H, strategy="relator_based", max_cosets=None, draft=None, incomplete=False): """ Return an instance of ``coset table``, when Todd-Coxeter algorithm is run over the ``self`` with ``H`` as subgroup, using ``strategy`` argument as strategy. The returned coset table is compressed but not standardized. An instance of `CosetTable` for `fp_grp` can be passed as the keyword argument `draft` in which case the coset enumeration will start with that instance and attempt to complete it. When `incomplete` is `True` and the function is unable to complete for some reason, the partially complete table will be returned. """ if not max_cosets: max_cosets = CosetTable.coset_table_max_limit if strategy == 'relator_based': C = coset_enumeration_r(self, H, max_cosets=max_cosets, draft=draft, incomplete=incomplete) else: C = coset_enumeration_c(self, H, max_cosets=max_cosets, draft=draft, incomplete=incomplete) if C.is_complete(): C.compress() return C def standardize_coset_table(self): """ Standardized the coset table ``self`` and makes the internal variable ``_is_standardized`` equal to ``True``. """ self._coset_table.standardize() self._is_standardized = True def coset_table(self, H, strategy="relator_based", max_cosets=None, draft=None, incomplete=False): """ Return the mathematical coset table of ``self`` in ``H``. """ if not H: if self._coset_table is not None: if not self._is_standardized: self.standardize_coset_table() else: C = self.coset_enumeration([], strategy, max_cosets=max_cosets, draft=draft, incomplete=incomplete) self._coset_table = C self.standardize_coset_table() return self._coset_table.table else: C = self.coset_enumeration(H, strategy, max_cosets=max_cosets, draft=draft, incomplete=incomplete) C.standardize() return C.table def order(self, strategy="relator_based"): """ Returns the order of the finitely presented group ``self``. It uses the coset enumeration with identity group as subgroup, i.e ``H=[]``. Examples ======== >>> from sympy.combinatorics.free_groups import free_group >>> from sympy.combinatorics.fp_groups import FpGroup >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x, y**2]) >>> f.order(strategy="coset_table_based") 2 """ from sympy import S, gcd if self._order is not None: return self._order if self._coset_table is not None: self._order = len(self._coset_table.table) elif len(self.relators) == 0: self._order = self.free_group.order() elif len(self.generators) == 1: self._order = abs(gcd([r.array_form[0][1] for r in self.relators])) elif self._is_infinite(): self._order = S.Infinity else: gens, C = self._finite_index_subgroup() if C: ind = len(C.table) self._order = ind*self.subgroup(gens, C=C).order() else: self._order = self.index([]) return self._order def _is_infinite(self): ''' Test if the group is infinite. Return `True` if the test succeeds and `None` otherwise ''' used_gens = set() for r in self.relators: used_gens.update(r.contains_generators()) if any([g not in used_gens for g in self.generators]): return True # Abelianisation test: check is the abelianisation is infinite abelian_rels = [] from sympy.polys.solvers import RawMatrix as Matrix from sympy.polys.domains import ZZ from sympy.matrices.normalforms import invariant_factors for rel in self.relators: abelian_rels.append([rel.exponent_sum(g) for g in self.generators]) m = Matrix(abelian_rels) setattr(m, "ring", ZZ) if 0 in invariant_factors(m): return True else: return None def _finite_index_subgroup(self, s=[]): ''' Find the elements of `self` that generate a finite index subgroup and, if found, return the list of elements and the coset table of `self` by the subgroup, otherwise return `(None, None)` ''' gen = self.most_frequent_generator() rels = list(self.generators) rels.extend(self.relators) if not s: if len(self.generators) == 2: s = [gen] + [g for g in self.generators if g != gen] else: rand = self.free_group.identity i = 0 while ((rand in rels or rand**-1 in rels or rand.is_identity) and i<10): rand = self.random() i += 1 s = [gen, rand] + [g for g in self.generators if g != gen] mid = (len(s)+1)//2 half1 = s[:mid] half2 = s[mid:] draft1 = None draft2 = None m = 200 C = None while not C and (m/2 < CosetTable.coset_table_max_limit): m = min(m, CosetTable.coset_table_max_limit) draft1 = self.coset_enumeration(half1, max_cosets=m, draft=draft1, incomplete=True) if draft1.is_complete(): C = draft1 half = half1 else: draft2 = self.coset_enumeration(half2, max_cosets=m, draft=draft2, incomplete=True) if draft2.is_complete(): C = draft2 half = half2 if not C: m *= 2 if not C: return None, None C.compress() return half, C def most_frequent_generator(self): gens = self.generators rels = self.relators freqs = [sum([r.generator_count(g) for r in rels]) for g in gens] return gens[freqs.index(max(freqs))] def random(self): import random r = self.free_group.identity for i in range(random.randint(2,3)): r = r*random.choice(self.generators)**random.choice([1,-1]) return r def index(self, H, strategy="relator_based"): """ Return the index of subgroup ``H`` in group ``self``. Examples ======== >>> from sympy.combinatorics.free_groups import free_group >>> from sympy.combinatorics.fp_groups import FpGroup >>> F, x, y = free_group("x, y") >>> f = FpGroup(F, [x**5, y**4, y*x*y**3*x**3]) >>> f.index([x]) 4 """ # TODO: use |G:H| = |G|/|H| (currently H can't be made into a group) # when we know |G| and |H| if H == []: return self.order() else: C = self.coset_enumeration(H, strategy) return len(C.table) def __str__(self): if self.free_group.rank > 30: str_form = "<fp group with %s generators>" % self.free_group.rank else: str_form = "<fp group on the generators %s>" % str(self.generators) return str_form __repr__ = __str__ #============================================================================== # PERMUTATION GROUP METHODS #============================================================================== def _to_perm_group(self): ''' Return an isomorphic permutation group and the isomorphism. The implementation is dependent on coset enumeration so will only terminate for finite groups. ''' from sympy.combinatorics import Permutation, PermutationGroup from sympy.combinatorics.homomorphisms import homomorphism if self.order() == S.Infinity: raise NotImplementedError("Permutation presentation of infinite " "groups is not implemented") if self._perm_isomorphism: T = self._perm_isomorphism P = T.image() else: C = self.coset_table([]) gens = self.generators images = [[C[i][2*gens.index(g)] for i in range(len(C))] for g in gens] images = [Permutation(i) for i in images] P = PermutationGroup(images) T = homomorphism(self, P, gens, images, check=False) self._perm_isomorphism = T return P, T def _perm_group_list(self, method_name, *args): ''' Given the name of a `PermutationGroup` method (returning a subgroup or a list of subgroups) and (optionally) additional arguments it takes, return a list or a list of lists containing the generators of this (or these) subgroups in terms of the generators of `self`. ''' P, T = self._to_perm_group() perm_result = getattr(P, method_name)(*args) single = False if isinstance(perm_result, PermutationGroup): perm_result, single = [perm_result], True result = [] for group in perm_result: gens = group.generators result.append(T.invert(gens)) return result[0] if single else result def derived_series(self): ''' Return the list of lists containing the generators of the subgroups in the derived series of `self`. ''' return self._perm_group_list('derived_series') def lower_central_series(self): ''' Return the list of lists containing the generators of the subgroups in the lower central series of `self`. ''' return self._perm_group_list('lower_central_series') def center(self): ''' Return the list of generators of the center of `self`. ''' return self._perm_group_list('center') def derived_subgroup(self): ''' Return the list of generators of the derived subgroup of `self`. ''' return self._perm_group_list('derived_subgroup') def centralizer(self, other): ''' Return the list of generators of the centralizer of `other` (a list of elements of `self`) in `self`. ''' T = self._to_perm_group()[1] other = T(other) return self._perm_group_list('centralizer', other) def normal_closure(self, other): ''' Return the list of generators of the normal closure of `other` (a list of elements of `self`) in `self`. ''' T = self._to_perm_group()[1] other = T(other) return self._perm_group_list('normal_closure', other) def _perm_property(self, attr): ''' Given an attribute of a `PermutationGroup`, return its value for a permutation group isomorphic to `self`. ''' P = self._to_perm_group()[0] return getattr(P, attr) @property def is_abelian(self): ''' Check if `self` is abelian. ''' return self._perm_property("is_abelian") @property def is_nilpotent(self): ''' Check if `self` is nilpotent. ''' return self._perm_property("is_nilpotent") @property def is_solvable(self): ''' Check if `self` is solvable. ''' return self._perm_property("is_solvable") @property def elements(self): ''' List the elements of `self`. ''' P, T = self._to_perm_group() return T.invert(P._elements) @property def is_cyclic(self): """ Return ``True`` if group is Cyclic. """ if len(self.generators) <= 1: return True try: P, T = self._to_perm_group() except NotImplementedError: raise NotImplementedError("Check for infinite Cyclic group " "is not implemented") return P.is_cyclic