Regular Crystals#

class sage.categories.regular_crystals.RegularCrystals#

Bases: Category_singleton

The category of regular crystals.

A crystal is called regular if every vertex \(b\) satisfies

\[\varepsilon_i(b) = \max\{ k \mid e_i^k(b) \neq 0 \} \quad \text{and} \quad \varphi_i(b) = \max\{ k \mid f_i^k(b) \neq 0 \}.\]

Note

Regular crystals are sometimes referred to as normal. When only one of the conditions (on either \(\varphi_i\) or \(\varepsilon_i\)) holds, these crystals are sometimes called seminormal or semiregular.

EXAMPLES:

sage: C = RegularCrystals()
sage: C
Category of regular crystals
sage: C.super_categories()
[Category of crystals]
sage: C.example()
Highest weight crystal of type A_3 of highest weight omega_1

class ElementMethods#

Bases: object

demazure_operator_simple(i, ring=None)#

Return the Demazure operator \(D_i\) applied to self.

INPUT:

i – an element of the index set of the underlying crystal
ring – (default: QQ) a ring

OUTPUT:

An element of the ring-free module indexed by the underlying crystal.

Let \(r = \langle \mathrm{wt}(b), \alpha^{\vee}_i \rangle\), then \(D_i(b)\) is defined as follows:

If \(r \geq 0\), this returns the sum of the elements obtained from self by application of \(f_i^k\) for \(0 \leq k \leq r\).
If \(r < 0\), this returns the opposite of the sum of the elements obtained by application of \(e_i^k\) for \(0 < k < -r\).

REFERENCES:

EXAMPLES:

sage: T = crystals.Tableaux(['A',2], shape=[2,1])
sage: t = T(rows=[[1,2],[2]])
sage: t.demazure_operator_simple(2)
B[[[1, 2], [2]]] + B[[[1, 3], [2]]] + B[[[1, 3], [3]]]
sage: t.demazure_operator_simple(2).parent()
Algebra of The crystal of tableaux of type ['A', 2] and shape(s) [[2, 1]]
        over Integer Ring

sage: t.demazure_operator_simple(1)
0

sage: K = crystals.KirillovReshetikhin(['A',2,1],2,1)
sage: t = K(rows=[[3],[2]])
sage: t.demazure_operator_simple(0)
B[[[1, 2]]] + B[[[2, 3]]]

dual_equivalence_class(index_set=None)#

Return the dual equivalence class indexed by index_set of self.

The dual equivalence class of an element \(b \in B\) is the set of all elements of \(B\) reachable from \(b\) via sequences of \(i\)-elementary dual equivalence relations (i.e., \(i\)-elementary dual equivalence transformations and their inverses) for \(i\) in the index set of \(B\).

For this to be well-defined, the element \(b\) has to be of weight \(0\) with respect to \(I\); that is, we need to have \(\varepsilon_j(b) = \varphi_j(b)\) for all \(j \in I\).

See [As2008]. See also dual_equivalence_graph() for a definition of \(i\)-elementary dual equivalence transformations.

INPUT:

index_set – (optional) the index set \(I\) (default: the whole index set of the crystal); this has to be a subset of the index set of the crystal (as a list or tuple)

OUTPUT:

The dual equivalence class of self indexed by the subset index_set. This class is returned as an undirected edge-colored multigraph. The color of an edge is the index \(i\) of the dual equivalence relation it encodes.

See also

EXAMPLES:

sage: T = crystals.Tableaux(['A',3], shape=[2,2])
sage: G = T(2,1,4,3).dual_equivalence_class()
sage: G.edges(sort=True)
[([[1, 3], [2, 4]], [[1, 2], [3, 4]], 2),
 ([[1, 3], [2, 4]], [[1, 2], [3, 4]], 3)]
sage: T = crystals.Tableaux(['A',4], shape=[3,2])
sage: G = T(2,1,4,3,5).dual_equivalence_class()
sage: G.edges(sort=True)
[([[1, 3, 5], [2, 4]], [[1, 3, 4], [2, 5]], 4),
 ([[1, 3, 5], [2, 4]], [[1, 2, 5], [3, 4]], 2),
 ([[1, 3, 5], [2, 4]], [[1, 2, 5], [3, 4]], 3),
 ([[1, 3, 4], [2, 5]], [[1, 2, 4], [3, 5]], 2),
 ([[1, 2, 4], [3, 5]], [[1, 2, 3], [4, 5]], 3),
 ([[1, 2, 4], [3, 5]], [[1, 2, 3], [4, 5]], 4)]

epsilon(i)#

Return \(\varepsilon_i\) of self.

EXAMPLES:

sage: C = crystals.Letters(['A',5])
sage: C(1).epsilon(1)
0
sage: C(2).epsilon(1)
1

phi(i)#

Return \(\varphi_i\) of self.

EXAMPLES:

sage: C = crystals.Letters(['A',5])
sage: C(1).phi(1)
1
sage: C(2).phi(1)
0

stembridgeDel_depth(i, j)#

Return the difference in the \(j\)-depth of self and \(f_i\) of self, where \(i\) and \(j\) are in the index set of the underlying crystal. This function is useful for checking the Stembridge local axioms for crystal bases.

The \(i\)-depth of a crystal node \(x\) is \(\varepsilon_i(x)\).

EXAMPLES:

sage: T = crystals.Tableaux(['A',2], shape=[2,1])
sage: t = T(rows=[[1,1],[2]])
sage: t.stembridgeDel_depth(1,2)
0
sage: s = T(rows=[[1,3],[3]])
sage: s.stembridgeDel_depth(1,2)
-1

stembridgeDel_rise(i, j)#

Return the difference in the \(j\)-rise of self and \(f_i\) of self, where \(i\) and \(j\) are in the index set of the underlying crystal. This function is useful for checking the Stembridge local axioms for crystal bases.

The \(i\)-rise of a crystal node \(x\) is \(\varphi_i(x)\).

EXAMPLES:

sage: T = crystals.Tableaux(['A',2], shape=[2,1])
sage: t = T(rows=[[1,1],[2]])
sage: t.stembridgeDel_rise(1,2)
-1
sage: s = T(rows=[[1,3],[3]])
sage: s.stembridgeDel_rise(1,2)
0

stembridgeDelta_depth(i, j)#

Return the difference in the \(j\)-depth of self and \(e_i\) of self, where \(i\) and \(j\) are in the index set of the underlying crystal. This function is useful for checking the Stembridge local axioms for crystal bases.

The \(i\)-depth of a crystal node \(x\) is \(-\varepsilon_i(x)\).

EXAMPLES:

sage: T = crystals.Tableaux(['A',2], shape=[2,1])
sage: t = T(rows=[[1,2],[2]])
sage: t.stembridgeDelta_depth(1,2)
0
sage: s = T(rows=[[2,3],[3]])
sage: s.stembridgeDelta_depth(1,2)
-1

stembridgeDelta_rise(i, j)#

Return the difference in the \(j\)-rise of self and \(e_i\) of self, where \(i\) and \(j\) are in the index set of the underlying crystal. This function is useful for checking the Stembridge local axioms for crystal bases.

The \(i\)-rise of a crystal node \(x\) is \(\varphi_i(x)\).

EXAMPLES:

sage: T = crystals.Tableaux(['A',2], shape=[2,1])
sage: t = T(rows=[[1,2],[2]])
sage: t.stembridgeDelta_rise(1,2)
-1
sage: s = T(rows=[[2,3],[3]])
sage: s.stembridgeDelta_rise(1,2)
0

stembridgeTriple(i, j)#

Let \(A\) be the Cartan matrix of the crystal, \(x\) a crystal element, and let \(i\) and \(j\) be in the index set of the crystal. Further, set b=stembridgeDelta_depth(x,i,j), and c=stembridgeDelta_rise(x,i,j)). If x.e(i) is non-empty, this function returns the triple \(( A_{ij}, b, c )\); otherwise it returns None. By the Stembridge local characterization of crystal bases, one should have \(A_{ij}=b+c\).

EXAMPLES:

sage: T = crystals.Tableaux(['A',2], shape=[2,1])
sage: t = T(rows=[[1,1],[2]])
sage: t.stembridgeTriple(1,2)
sage: s = T(rows=[[1,2],[2]])
sage: s.stembridgeTriple(1,2)
(-1, 0, -1)

sage: T = crystals.Tableaux(['B',2], shape=[2,1])
sage: t = T(rows=[[1,2],[2]])
sage: t.stembridgeTriple(1,2)
(-2, 0, -2)
sage: s = T(rows=[[-1,-1],[0]])
sage: s.stembridgeTriple(1,2)
(-2, -2, 0)
sage: u = T(rows=[[0,2],[1]])
sage: u.stembridgeTriple(1,2)
(-2, -1, -1)

weight()#

Return the weight of this crystal element.

EXAMPLES:

sage: C = crystals.Letters(['A',5])
sage: C(1).weight()
(1, 0, 0, 0, 0, 0)

class MorphismMethods#

Bases: object

is_isomorphism()#

Check if self is a crystal isomorphism, which is true if and only if this is a strict embedding with the same number of connected components.

EXAMPLES:

sage: A21 = RootSystem(['A',2,1])
sage: La = A21.weight_space(extended=True).fundamental_weights()
sage: B = crystals.LSPaths(La[0])
sage: La = A21.weight_lattice(extended=True).fundamental_weights()
sage: C = crystals.GeneralizedYoungWalls(2, La[0])
sage: H = Hom(B, C)
sage: from sage.categories.highest_weight_crystals import HighestWeightCrystalMorphism
sage: class Psi(HighestWeightCrystalMorphism):
....:     def is_strict(self):
....:         return True
sage: psi = Psi(H, C.module_generators); psi
['A', 2, 1] Crystal morphism:
  From: The crystal of LS paths of type ['A', 2, 1] and weight Lambda[0]
  To:   Highest weight crystal of generalized Young walls
        of Cartan type ['A', 2, 1] and highest weight Lambda[0]
  Defn: (Lambda[0],) |--> []
sage: psi.is_isomorphism()
True

class ParentMethods#

Bases: object

demazure_operator(element, reduced_word)#

Returns the application of Demazure operators \(D_i\) for \(i\) from reduced_word on element.

INPUT:

element – an element of a free module indexed by the underlying crystal
reduced_word – a reduced word of the Weyl group of the same type as the underlying crystal

OUTPUT:

an element of the free module indexed by the underlying crystal

EXAMPLES:

sage: T = crystals.Tableaux(['A',2], shape=[2,1])
sage: C = CombinatorialFreeModule(QQ, T)
sage: t = T.highest_weight_vector()
sage: b = 2*C(t)
sage: T.demazure_operator(b,[1,2,1])
2*B[[[1, 1], [2]]] + 2*B[[[1, 2], [2]]] + 2*B[[[1, 3], [2]]]
+ 2*B[[[1, 1], [3]]] + 2*B[[[1, 2], [3]]] + 2*B[[[1, 3], [3]]]
+ 2*B[[[2, 2], [3]]] + 2*B[[[2, 3], [3]]]

The Demazure operator is idempotent:

sage: T = crystals.Tableaux("A1", shape=[4])
sage: C = CombinatorialFreeModule(QQ, T)
sage: b = C(T.module_generators[0]); b
B[[[1, 1, 1, 1]]]
sage: e = T.demazure_operator(b,[1]); e
B[[[1, 1, 1, 1]]] + B[[[1, 1, 1, 2]]] + B[[[1, 1, 2, 2]]]
+ B[[[1, 2, 2, 2]]] + B[[[2, 2, 2, 2]]]
sage: e == T.demazure_operator(e,[1])
True

sage: all(T.demazure_operator(T.demazure_operator(C(t),[1]),[1])
....:      == T.demazure_operator(C(t),[1]) for t in T)
True

demazure_subcrystal(element, reduced_word, only_support=True)#

Return the subcrystal corresponding to the application of Demazure operators \(D_i\) for \(i\) from reduced_word on element.

INPUT:

element – an element of a free module indexed by the underlying crystal
reduced_word – a reduced word of the Weyl group of the same type as the underlying crystal
only_support – (default: True) only include arrows corresponding to the support of reduced_word

OUTPUT:

the Demazure subcrystal

EXAMPLES:

sage: T = crystals.Tableaux(['A',2], shape=[2,1])
sage: t = T.highest_weight_vector()
sage: S = T.demazure_subcrystal(t, [1,2])
sage: list(S)
[[[1, 1], [2]], [[1, 2], [2]], [[1, 1], [3]],
 [[1, 2], [3]], [[2, 2], [3]]]
sage: S = T.demazure_subcrystal(t, [2,1])
sage: list(S)
[[[1, 1], [2]], [[1, 2], [2]], [[1, 1], [3]],
 [[1, 3], [2]], [[1, 3], [3]]]

We construct an example where we don’t only want the arrows indicated by the support of the reduced word:

sage: K = crystals.KirillovReshetikhin(['A',1,1], 1, 2)
sage: mg = K.module_generator()
sage: S = K.demazure_subcrystal(mg, [1])
sage: S.digraph().edges(sort=True)
[([[1, 1]], [[1, 2]], 1), ([[1, 2]], [[2, 2]], 1)]
sage: S = K.demazure_subcrystal(mg, [1], only_support=False)
sage: S.digraph().edges(sort=True)
[([[1, 1]], [[1, 2]], 1),
 ([[1, 2]], [[1, 1]], 0),
 ([[1, 2]], [[2, 2]], 1),
 ([[2, 2]], [[1, 2]], 0)]

dual_equivalence_graph(X=None, index_set=None, directed=True)#

Return the dual equivalence graph indexed by index_set on the subset X of self.

Let \(b \in B\) be an element of weight \(0\), so \(\varepsilon_j(b) = \varphi_j(b)\) for all \(j \in I\), where \(I\) is the indexing set. We say \(b'\) is an \(i\)-elementary dual equivalence transformation of \(b\) (where \(i \in I\)) if

\(\varepsilon_i(b) = 1\) and \(\varepsilon_{i-1}(b) = 0\), and
\(b' = f_{i-1} f_i e_{i-1} e_i b\).

We can do the inverse procedure by interchanging \(i\) and \(i-1\) above.

Note

If the index set is not an ordered interval, we let \(i - 1\) mean the index appearing before \(i\) in \(I\).

This definition comes from [As2008] Section 4 (where our \(\varphi_j(b)\) and \(\varepsilon_j(b)\) are denoted by \(\epsilon(b, j)\) and \(-\delta(b, j)\), respectively).

The dual equivalence graph of \(B\) is defined to be the colored graph whose vertices are the elements of \(B\) of weight \(0\), and whose edges of color \(i\) (for \(i \in I\)) connect pairs \(\{ b, b' \}\) such that \(b'\) is an \(i\)-elementary dual equivalence transformation of \(b\).

Note

This dual equivalence graph is a generalization of \(\mathcal{G}\left(\mathcal{X}\right)\) in [As2008] Section 4 except we do not require \(\varepsilon_i(b) = 0, 1\) for all \(i\).

This definition can be generalized by choosing a subset \(X\) of the set of all vertices of \(B\) of weight \(0\), and restricting the dual equivalence graph to the vertex set \(X\).

INPUT:

X – (optional) the vertex set \(X\) (default: the whole set of vertices of self of weight \(0\))
index_set – (optional) the index set \(I\) (default: the whole index set of self); this has to be a subset of the index set of self (as a list or tuple)
directed – (default: True) whether to have the dual equivalence graph be directed, where the head of an edge \(b - b'\) is \(b\) and the tail is \(b' = f_{i-1} f_i e_{i-1} e_i b\))