# Highest Weight Crystals¶

class sage.categories.highest_weight_crystals.HighestWeightCrystalHomset(X, Y, category=None)

The set of crystal morphisms from a highest weight crystal to another crystal.

See sage.categories.crystals.CrystalHomset for more information.

Element
class sage.categories.highest_weight_crystals.HighestWeightCrystalMorphism(parent, on_gens, cartan_type=None, virtualization=None, scaling_factors=None, gens=None, check=True)

A virtual crystal morphism whose domain is a highest weight crystal.

INPUT:

• parent – a homset

• on_gens – a function or list that determines the image of the generators (if given a list, then this uses the order of the generators of the domain) of the domain under self

• cartan_type – (optional) a Cartan type; the default is the Cartan type of the domain

• virtualization – (optional) a dictionary whose keys are in the index set of the domain and whose values are lists of entries in the index set of the codomain

• scaling_factors – (optional) a dictionary whose keys are in the index set of the domain and whose values are scaling factors for the weight, $$\varepsilon$$ and $$\varphi$$

• gens – (optional) a list of generators to define the morphism; the default is to use the highest weight vectors of the crystal

• check – (default: True) check if the crystal morphism is valid

class sage.categories.highest_weight_crystals.HighestWeightCrystals(s=None)

The category of highest weight crystals.

A crystal is highest weight if it is acyclic; in particular, every connected component has a unique highest weight element, and that element generate the component.

EXAMPLES:

sage: C = HighestWeightCrystals()
sage: C
Category of highest weight 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

string_parameters(word=None)

Return the string parameters of self corresponding to the reduced word word.

Given a reduced expression $$w = s_{i_1} \cdots s_{i_k}$$, the string parameters of $$b \in B$$ corresponding to $$w$$ are $$(a_1, \ldots, a_k)$$ such that

\begin{split}\begin{aligned} e_{i_m}^{a_m} \cdots e_{i_1}^{a_1} b & \neq 0 \\ e_{i_m}^{a_m+1} \cdots e_{i_1}^{a_1} b & = 0 \end{aligned}\end{split}

for all $$1 \leq m \leq k$$.

For connected components isomorphic to $$B(\lambda)$$ or $$B(\infty)$$, if $$w = w_0$$ is the longest element of the Weyl group, then the path determined by the string parametrization terminates at the highest weight vector.

INPUT:

• word – a word in the alphabet of the index set; if not specified and we are in finite type, then this will be some reduced expression for the long element determined by the Weyl group

EXAMPLES:

sage: B = crystals.infinity.NakajimaMonomials(['A',3])
sage: mg = B.highest_weight_vector()
sage: w0 = [1,2,1,3,2,1]
sage: mg.string_parameters(w0)
[0, 0, 0, 0, 0, 0]
sage: mg.f_string([1]).string_parameters(w0)
[1, 0, 0, 0, 0, 0]
sage: mg.f_string([1,1,1]).string_parameters(w0)
[3, 0, 0, 0, 0, 0]
sage: mg.f_string([1,1,1,2,2]).string_parameters(w0)
[1, 2, 2, 0, 0, 0]
sage: mg.f_string([1,1,1,2,2]) == mg.f_string([1,1,2,2,1])
True
sage: x = mg.f_string([1,1,1,2,2,1,3,3,2,1,1,1])
sage: x.string_parameters(w0)
[4, 1, 1, 2, 2, 2]
sage: x.string_parameters([3,2,1,3,2,3])
[2, 3, 7, 0, 0, 0]
sage: x == mg.f_string([1]*7 + [2]*3 + [3]*2)
True

sage: B = crystals.infinity.Tableaux("A5")
sage: b = B(rows=[[1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,3,6,6,6,6,6,6],
....:             [2,2,2,2,2,2,2,2,2,4,5,5,5,6],
....:             [3,3,3,3,3,3,3,5],
....:             [4,4,4,6,6,6],
....:             [5,6]])
sage: b.string_parameters([1,2,1,3,2,1,4,3,2,1,5,4,3,2,1])
[0, 1, 1, 1, 1, 0, 4, 4, 3, 0, 11, 10, 7, 7, 6]

sage: B = crystals.infinity.Tableaux("G2")
sage: b = B(rows=[[1,1,1,1,1,3,3,0,-3,-3,-2,-2,-1,-1,-1,-1],[2,3,3,3]])
sage: b.string_parameters([2,1,2,1,2,1])
[5, 13, 11, 15, 4, 4]
sage: b.string_parameters([1,2,1,2,1,2])
[7, 12, 15, 8, 10, 0]

sage: C = crystals.Tableaux(['C',2], shape=[2,1])
sage: mg = C.highest_weight_vector()
sage: lw = C.lowest_weight_vectors()[0]
sage: lw.string_parameters([1,2,1,2])
[1, 2, 3, 1]
sage: lw.string_parameters([2,1,2,1])
[1, 3, 2, 1]
sage: lw.e_string([2,1,1,1,2,2,1]) == mg
True
sage: lw.e_string([1,2,2,1,1,1,2]) == mg
True

class ParentMethods

Bases: object

connected_components_generators()

Returns the highest weight vectors of self

This default implementation selects among the module generators those that are highest weight, and caches the result. A crystal element $$b$$ is highest weight if $$e_i(b)=0$$ for all $$i$$ in the index set.

EXAMPLES:

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

sage: C = crystals.Letters(['A',2])
sage: T = crystals.TensorProduct(C,C,C,generators=[[C(2),C(1),C(1)],[C(1),C(2),C(1)]])
sage: T.highest_weight_vectors()
([2, 1, 1], [1, 2, 1])

digraph(subset=None, index_set=None, depth=None)

Return the DiGraph associated to self.

INPUT:

• subset – (optional) a subset of vertices for which the digraph should be constructed

• index_set – (optional) the index set to draw arrows

• depth – the depth to draw; optional only for finite crystals

EXAMPLES:

sage: T = crystals.Tableaux(['A',2], shape=[2,1])
sage: T.digraph()
Digraph on 8 vertices
sage: S = T.subcrystal(max_depth=2)
sage: len(S)
5
sage: G = T.digraph(subset=list(S))
sage: G.is_isomorphic(T.digraph(depth=2), edge_labels=True)
True

highest_weight_vector()

Returns the highest weight vector if there is a single one; otherwise, raises an error.

Caveat: this assumes that highest_weight_vectors() returns a list or tuple.

EXAMPLES:

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

highest_weight_vectors()

Returns the highest weight vectors of self

This default implementation selects among the module generators those that are highest weight, and caches the result. A crystal element $$b$$ is highest weight if $$e_i(b)=0$$ for all $$i$$ in the index set.

EXAMPLES:

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

sage: C = crystals.Letters(['A',2])
sage: T = crystals.TensorProduct(C,C,C,generators=[[C(2),C(1),C(1)],[C(1),C(2),C(1)]])
sage: T.highest_weight_vectors()
([2, 1, 1], [1, 2, 1])

lowest_weight_vectors()

Return the lowest weight vectors of self.

This default implementation selects among all elements of the crystal those that are lowest weight, and cache the result. A crystal element $$b$$ is lowest weight if $$f_i(b)=0$$ for all $$i$$ in the index set.

EXAMPLES:

sage: C = crystals.Letters(['A',5])
sage: C.lowest_weight_vectors()
(6,)

sage: C = crystals.Letters(['A',2])
sage: T = crystals.TensorProduct(C,C,C,generators=[[C(2),C(1),C(1)],[C(1),C(2),C(1)]])
sage: T.lowest_weight_vectors()
([3, 2, 3], [3, 3, 2])

q_dimension(q=None, prec=None, use_product=False)

Return the $$q$$-dimension of self.

Let $$B(\lambda)$$ denote a highest weight crystal. Recall that the degree of the $$\mu$$-weight space of $$B(\lambda)$$ (under the principal gradation) is equal to $$\langle \rho^{\vee}, \lambda - \mu \rangle$$ where $$\langle \rho^{\vee}, \alpha_i \rangle = 1$$ for all $$i \in I$$ (in particular, take $$\rho^{\vee} = \sum_{i \in I} h_i$$).

The $$q$$-dimension of a highest weight crystal $$B(\lambda)$$ is defined as

$\dim_q B(\lambda) := \sum_{j \geq 0} \dim(B_j) q^j,$

where $$B_j$$ denotes the degree $$j$$ portion of $$B(\lambda)$$. This can be expressed as the product

$\dim_q B(\lambda) = \prod_{\alpha^{\vee} \in \Delta_+^{\vee}} \left( \frac{1 - q^{\langle \lambda + \rho, \alpha^{\vee} \rangle}}{1 - q^{\langle \rho, \alpha^{\vee} \rangle}} \right)^{\mathrm{mult}\, \alpha},$

where $$\Delta_+^{\vee}$$ denotes the set of positive coroots. Taking the limit as $$q \to 1$$ gives the dimension of $$B(\lambda)$$. For more information, see [Ka1990] Section 10.10.

INPUT:

• q – the (generic) parameter $$q$$

• prec – (default: None) The precision of the power series ring to use if the crystal is not known to be finite (i.e. the number of terms returned). If None, then the result is returned as a lazy power series.

• use_product – (default: False) if we have a finite crystal and True, use the product formula

EXAMPLES:

sage: C = crystals.Tableaux(['A',2], shape=[2,1])
sage: qdim = C.q_dimension(); qdim
q^4 + 2*q^3 + 2*q^2 + 2*q + 1
sage: qdim(1)
8
sage: len(C) == qdim(1)
True
sage: C.q_dimension(use_product=True) == qdim
True
sage: C.q_dimension(prec=20)
q^4 + 2*q^3 + 2*q^2 + 2*q + 1
sage: C.q_dimension(prec=2)
2*q + 1

sage: R.<t> = QQ[]
sage: C.q_dimension(q=t^2)
t^8 + 2*t^6 + 2*t^4 + 2*t^2 + 1

sage: C = crystals.Tableaux(['A',2], shape=[5,2])
sage: C.q_dimension()
q^10 + 2*q^9 + 4*q^8 + 5*q^7 + 6*q^6 + 6*q^5
+ 6*q^4 + 5*q^3 + 4*q^2 + 2*q + 1

sage: C = crystals.Tableaux(['B',2], shape=[2,1])
sage: qdim = C.q_dimension(); qdim
q^10 + 2*q^9 + 3*q^8 + 4*q^7 + 5*q^6 + 5*q^5
+ 5*q^4 + 4*q^3 + 3*q^2 + 2*q + 1
sage: qdim == C.q_dimension(use_product=True)
True

sage: C = crystals.Tableaux(['D',4], shape=[2,1])
sage: C.q_dimension()
q^16 + 2*q^15 + 4*q^14 + 7*q^13 + 10*q^12 + 13*q^11
+ 16*q^10 + 18*q^9 + 18*q^8 + 18*q^7 + 16*q^6 + 13*q^5
+ 10*q^4 + 7*q^3 + 4*q^2 + 2*q + 1


We check with a finite tensor product:

sage: TP = crystals.TensorProduct(C, C)
sage: TP.cardinality()
25600
sage: qdim = TP.q_dimension(use_product=True); qdim # long time
q^32 + 2*q^31 + 8*q^30 + 15*q^29 + 34*q^28 + 63*q^27 + 110*q^26
+ 175*q^25 + 276*q^24 + 389*q^23 + 550*q^22 + 725*q^21
+ 930*q^20 + 1131*q^19 + 1362*q^18 + 1548*q^17 + 1736*q^16
+ 1858*q^15 + 1947*q^14 + 1944*q^13 + 1918*q^12 + 1777*q^11
+ 1628*q^10 + 1407*q^9 + 1186*q^8 + 928*q^7 + 720*q^6
+ 498*q^5 + 342*q^4 + 201*q^3 + 117*q^2 + 48*q + 26
sage: qdim(1) # long time
25600
sage: TP.q_dimension() == qdim # long time
True


The $$q$$-dimensions of infinite crystals are returned as formal power series:

sage: C = crystals.LSPaths(['A',2,1], [1,0,0])
sage: C.q_dimension(prec=5)
1 + q + 2*q^2 + 2*q^3 + 4*q^4 + O(q^5)
sage: C.q_dimension(prec=10)
1 + q + 2*q^2 + 2*q^3 + 4*q^4 + 5*q^5 + 7*q^6
+ 9*q^7 + 13*q^8 + 16*q^9 + O(q^10)
sage: qdim = C.q_dimension(); qdim
1 + q + 2*q^2 + 2*q^3 + 4*q^4 + 5*q^5 + 7*q^6
+ 9*q^7 + 13*q^8 + 16*q^9 + 22*q^10 + O(x^11)
sage: qdim.compute_coefficients(15)
sage: qdim
1 + q + 2*q^2 + 2*q^3 + 4*q^4 + 5*q^5 + 7*q^6
+ 9*q^7 + 13*q^8 + 16*q^9 + 22*q^10 + 27*q^11
+ 36*q^12 + 44*q^13 + 57*q^14 + 70*q^15 + O(x^16)

class TensorProducts(category, *args)

The category of highest weight crystals constructed by tensor product of highest weight crystals.

class ParentMethods

Bases: object

Implements operations on tensor products of crystals.

highest_weight_vectors()

Return the highest weight vectors of self.

This works by using a backtracing algorithm since if $$b_2 \otimes b_1$$ is highest weight then $$b_1$$ is highest weight.

EXAMPLES:

sage: C = crystals.Tableaux(['D',4], shape=[2,2])
sage: D = crystals.Tableaux(['D',4], shape=[1])
sage: T = crystals.TensorProduct(D, C)
sage: T.highest_weight_vectors()
([[[1]], [[1, 1], [2, 2]]],
[[[3]], [[1, 1], [2, 2]]],
[[[-2]], [[1, 1], [2, 2]]])
sage: L = filter(lambda x: x.is_highest_weight(), T)
sage: tuple(L) == T.highest_weight_vectors()
True

highest_weight_vectors_iterator()

Iterate over the highest weight vectors of self.

This works by using a backtracing algorithm since if $$b_2 \otimes b_1$$ is highest weight then $$b_1$$ is highest weight.

EXAMPLES:

sage: C = crystals.Tableaux(['D',4], shape=[2,2])
sage: D = crystals.Tableaux(['D',4], shape=[1])
sage: T = crystals.TensorProduct(D, C)
sage: tuple(T.highest_weight_vectors_iterator())
([[[1]], [[1, 1], [2, 2]]],
[[[3]], [[1, 1], [2, 2]]],
[[[-2]], [[1, 1], [2, 2]]])
sage: L = filter(lambda x: x.is_highest_weight(), T)
sage: tuple(L) == tuple(T.highest_weight_vectors_iterator())
True

extra_super_categories()

EXAMPLES:

sage: HighestWeightCrystals().TensorProducts().extra_super_categories()
[Category of highest weight crystals]


Return None.

Indeed, the category of highest weight crystals defines no additional structure: it only guarantees the existence of a unique highest weight element in each component.

Todo

Should this category be a CategoryWithAxiom?

EXAMPLES:

sage: HighestWeightCrystals().additional_structure()

example()

Returns an example of highest weight crystals, as per Category.example().

EXAMPLES:

sage: B = HighestWeightCrystals().example(); B
Highest weight crystal of type A_3 of highest weight omega_1

super_categories()

EXAMPLES:

sage: HighestWeightCrystals().super_categories()
[Category of crystals]