Utilities¶
- sympy.combinatorics.util._base_ordering(base, degree)[source]¶
Order \(\{0, 1, ..., n-1\}\) so that base points come first and in order.
Parameters : ``base`` - the base :
``degree`` - the degree of the associated permutation group :
Returns : A list ``base_ordering`` such that ``base_ordering[point]`` is the :
number of ``point`` in the ordering. :
Examples :
======== :
>>> from sympy.combinatorics.named_groups import SymmetricGroup :
>>> from sympy.combinatorics.util import _base_ordering :
>>> S = SymmetricGroup(4) :
>>> S.schreier_sims() :
>>> _base_ordering(S.base, S.degree) :
[0, 1, 2, 3] :
Notes
This is used in backtrack searches, when we define a relation \(<<\) on the underlying set for a permutation group of degree \(n\), \(\{0, 1, ..., n-1\}\), so that if \((b_1, b_2, ..., b_k)\) is a base we have \(b_i << b_j\) whenever \(i<j\) and \(b_i << a\) for all \(i\in\{1,2, ..., k\}\) and \(a\) is not in the base. The idea is developed and applied to backtracking algorithms in [1], pp.108-132. The points that are not in the base are taken in increasing order.
References
[1] Holt, D., Eick, B., O’Brien, E. “Handbook of computational group theory”
- sympy.combinatorics.util._check_cycles_alt_sym(perm)[source]¶
Checks for cycles of prime length p with n/2 < p < n-2.
Here \(n\) is the degree of the permutation. This is a helper function for the function is_alt_sym from sympy.combinatorics.perm_groups.
Examples
>>> from sympy.combinatorics.util import _check_cycles_alt_sym >>> from sympy.combinatorics.permutations import Permutation >>> a = Permutation([[0,1,2,3,4,5,6,7,8,9,10], [11, 12]]) >>> _check_cycles_alt_sym(a) False >>> b = Permutation([[0,1,2,3,4,5,6], [7,8,9,10]]) >>> _check_cycles_alt_sym(b) True
- sympy.combinatorics.util._distribute_gens_by_base(base, gens)[source]¶
Distribute the group elements gens by membership in basic stabilizers.
Notice that for a base \((b_1, b_2, ..., b_k)\), the basic stabilizers are defined as \(G^{(i)} = G_{b_1, ..., b_{i-1}}\) for \(i \in\{1, 2, ..., k\}\).
Parameters : ``base`` - a sequence of points in `{0, 1, ..., n-1}` :
``gens`` - a list of elements of a permutation group of degree `n`. :
Returns : List of length `k`, where `k` is :
the length of ``base``. The `i`-th entry contains those elements in :
``gens`` which fix the first `i` elements of ``base`` (so that the :
`0`-th entry is equal to ``gens`` itself). If no element fixes the first :
`i` elements of ``base``, the `i`-th element is set to a list containing :
the identity element. :
Examples
>>> from sympy.combinatorics import Permutation >>> Permutation.print_cyclic = True >>> from sympy.combinatorics.named_groups import DihedralGroup >>> from sympy.combinatorics.util import _distribute_gens_by_base >>> D = DihedralGroup(3) >>> D.schreier_sims() >>> D.strong_gens [Permutation(0, 1, 2), Permutation(0, 2), Permutation(1, 2)] >>> D.base [0, 1] >>> _distribute_gens_by_base(D.base, D.strong_gens) [[Permutation(0, 1, 2), Permutation(0, 2), Permutation(1, 2)], [Permutation(1, 2)]]
- sympy.combinatorics.util._handle_precomputed_bsgs(base, strong_gens, transversals=None, basic_orbits=None, strong_gens_distr=None)[source]¶
Calculate BSGS-related structures from those present.
The base and strong generating set must be provided; if any of the transversals, basic orbits or distributed strong generators are not provided, they will be calculated from the base and strong generating set.
Parameters : ``base`` - the base :
``strong_gens`` - the strong generators :
``transversals`` - basic transversals :
``basic_orbits`` - basic orbits :
``strong_gens_distr`` - strong generators distributed by membership in basic :
stabilizers :
Returns : ``(transversals, basic_orbits, strong_gens_distr)`` where ``transversals`` :
are the basic transversals, ``basic_orbits`` are the basic orbits, and :
``strong_gens_distr`` are the strong generators distributed by membership :
in basic stabilizers. :
See also
_orbits_transversals_from_bsgs, distribute_gens_by_base
Examples
>>> from sympy.combinatorics import Permutation >>> Permutation.print_cyclic = True >>> from sympy.combinatorics.named_groups import DihedralGroup >>> from sympy.combinatorics.util import _handle_precomputed_bsgs >>> D = DihedralGroup(3) >>> D.schreier_sims() >>> _handle_precomputed_bsgs(D.base, D.strong_gens, ... basic_orbits=D.basic_orbits) ([{0: Permutation(2), 1: Permutation(0, 1, 2), 2: Permutation(0, 2)}, {1: Permutation(2), 2: Permutation(1, 2)}], [[0, 1, 2], [1, 2]], [[Permutation(0, 1, 2), Permutation(0, 2), Permutation(1, 2)], [Permutation(1, 2)]])
- sympy.combinatorics.util._orbits_transversals_from_bsgs(base, strong_gens_distr, transversals_only=False)[source]¶
Compute basic orbits and transversals from a base and strong generating set.
The generators are provided as distributed across the basic stabilizers. If the optional argument transversals_only is set to True, only the transversals are returned.
Parameters : ``base`` - the base :
``strong_gens_distr`` - strong generators distributed by membership in basic :
stabilizers :
``transversals_only`` - a flag swithing between returning only the :
transversals/ both orbits and transversals :
Examples
>>> from sympy.combinatorics import Permutation >>> Permutation.print_cyclic = True >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> from sympy.combinatorics.util import _orbits_transversals_from_bsgs >>> from sympy.combinatorics.util import (_orbits_transversals_from_bsgs, ... _distribute_gens_by_base) >>> S = SymmetricGroup(3) >>> S.schreier_sims() >>> strong_gens_distr = _distribute_gens_by_base(S.base, S.strong_gens) >>> _orbits_transversals_from_bsgs(S.base, strong_gens_distr) ([[0, 1, 2], [1, 2]], [{0: Permutation(2), 1: Permutation(0, 1, 2), 2: Permutation(0, 2, 1)}, {1: Permutation(2), 2: Permutation(1, 2)}])
- sympy.combinatorics.util._remove_gens(base, strong_gens, basic_orbits=None, strong_gens_distr=None)[source]¶
Remove redundant generators from a strong generating set.
Parameters : ``base`` - a base :
``strong_gens`` - a strong generating set relative to ``base`` :
``basic_orbits`` - basic orbits :
``strong_gens_distr`` - strong generators distributed by membership in basic :
stabilizers :
Returns : A strong generating set with respect to ``base`` which is a subset of :
``strong_gens``. :
Notes
This procedure is outlined in [1],p.95.
References
[1] Holt, D., Eick, B., O’Brien, E. “Handbook of computational group theory”
Examples
>>> from sympy.combinatorics.named_groups import SymmetricGroup >>> from sympy.combinatorics.perm_groups import PermutationGroup >>> from sympy.combinatorics.util import _remove_gens >>> from sympy.combinatorics.testutil import _verify_bsgs >>> S = SymmetricGroup(15) >>> base, strong_gens = S.schreier_sims_incremental() >>> len(strong_gens) 26 >>> new_gens = _remove_gens(base, strong_gens) >>> len(new_gens) 14 >>> _verify_bsgs(S, base, new_gens) True
- sympy.combinatorics.util._strip(g, base, orbits, transversals)[source]¶
Attempt to decompose a permutation using a (possibly partial) BSGS structure.
This is done by treating the sequence base as an actual base, and the orbits orbits and transversals transversals as basic orbits and transversals relative to it.
This process is called “sifting”. A sift is unsuccessful when a certain orbit element is not found or when after the sift the decomposition doesn’t end with the identity element.
The argument transversals is a list of dictionaries that provides transversal elements for the orbits orbits.
Parameters : ``g`` - permutation to be decomposed :
``base`` - sequence of points :
``orbits`` - a list in which the ``i``-th entry is an orbit of ``base[i]`` :
under some subgroup of the pointwise stabilizer of ` :
`base[0], base[1], ..., base[i - 1]``. The groups themselves are implicit :
in this function since the only infromation we need is encoded in the orbits :
and transversals :
``transversals`` - a list of orbit transversals associated with the orbits :
``orbits``. :
See also
sympy.combinatorics.perm_groups.PermutationGroup.schreier_sims, sympy.combinatorics.perm_groups.PermutationGroup.schreier_sims_random
Notes
The algorithm is described in [1],pp.89-90. The reason for returning both the current state of the element being decomposed and the level at which the sifting ends is that they provide important information for the randomized version of the Schreier-Sims algorithm.
References
[1] Holt, D., Eick, B., O’Brien, E. “Handbook of computational group theory”
Examples
>>> from sympy.combinatorics import Permutation >>> Permutation.print_cyclic = True >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> from sympy.combinatorics.permutations import Permutation >>> from sympy.combinatorics.util import _strip >>> S = SymmetricGroup(5) >>> S.schreier_sims() >>> g = Permutation([0, 2, 3, 1, 4]) >>> _strip(g, S.base, S.basic_orbits, S.basic_transversals) (Permutation(4), 5)
- sympy.combinatorics.util._strong_gens_from_distr(strong_gens_distr)[source]¶
Retrieve strong generating set from generators of basic stabilizers.
This is just the union of the generators of the first and second basic stabilizers.
Parameters : ``strong_gens_distr`` - strong generators distributed by membership in basic :
stabilizers :
See also
Examples
>>> from sympy.combinatorics import Permutation >>> Permutation.print_cyclic = True >>> from sympy.combinatorics.named_groups import SymmetricGroup >>> from sympy.combinatorics.util import (_strong_gens_from_distr, ... _distribute_gens_by_base) >>> S = SymmetricGroup(3) >>> S.schreier_sims() >>> S.strong_gens [Permutation(0, 1, 2), Permutation(2)(0, 1), Permutation(1, 2)] >>> strong_gens_distr = _distribute_gens_by_base(S.base, S.strong_gens) >>> _strong_gens_from_distr(strong_gens_distr) [Permutation(0, 1, 2), Permutation(2)(0, 1), Permutation(1, 2)]
