Kyoto Path Model for Affine Highest Weight Crystals

class sage.combinat.crystals.kyoto_path_model.KyotoPathModel(crystals, weight)

Bases: sage.combinat.crystals.tensor_product.TensorProductOfCrystals

The Kyoto path model for an affine highest weight crystal.

Note

Here we are using anti-Kashiwara notation and might differ from some of the literature.

Consider a Kac–Moody algebra \mathfrak{g} of affine Cartan type X, and we want to model the U_q(\mathfrak{g})-crystal B(\lambda). First we consider the set of fundamental weights \{\Lambda_i\}_{i \in I} of \mathfrak{g} and let \{\overline{\Lambda}_i\}_{i \in I_0} be the corresponding fundamental weights of the corresponding classical Lie algebra \mathfrak{g}_0. To model B(\lambda), we start with a sequence of perfect U_q^{\prime}(\mathfrak{g})-crystals (B^{(i)})_i of level l such that

\lambda \in \overline{P}_l^+ = \left\{ \mu \in \overline{P}^+ \mid
\langle c, \mu \rangle = l \right\}

where c is the canonical central element of U_q(\mathfrak{g}) and \overline{P}^+ is the nonnegative weight lattice spanned by \{ \overline{\Lambda}_i \mid i \in I \}.

Next we consider the crystal isomorphism \Phi_0 : B(\lambda_0) \to B^{(0)}
\otimes B(\lambda_1) defined by u_{\lambda_0} \mapsto b^{(0)}_{\lambda_0}
\otimes u_{\lambda_1} where b^{(0)}_{\lambda_0} is the unique element in B^{(0)} such that \varphi\left( b^{(0)}_{\lambda_0} \right) = \lambda_0 and \lambda_1 = \varepsilon\left( b^{(0)}_{\lambda_0} \right) and u_{\mu} is the highest weight element in B(\mu). Iterating this, we obtain the following isomorphism:

\Phi_n : B(\lambda) \to B^{(0)} \otimes B^{(1)} \otimes \cdots
\otimes B^{(N)} \otimes B(\lambda_{N+1}).

We note by Lemma 10.6.2 in [HK02] that for any b \in B(\lambda) there exists a finite N such that

\Phi_N(b) = \left( \bigotimes_{k=0}^{N-1} b^{(k)} \right)
\otimes u_{\lambda_N}.

Therefore we can model elements b \in B(\lambda) as a U_q^{\prime}(\mathfrak{g})-crystal by considering an infinite list of elements b^{(k)} \in B^{(k)} and defining the crystal structure by:

\begin{aligned}
\overline{\mathrm{wt}}(b) & = \lambda_N + \sum_{k=0}^{N-1}
\overline{\mathrm{wt}}\left( b^{(k)} \right)
\\ e_i(b) & = e_i\left( b^{\prime} \otimes b^{(N)} \right) \otimes
u_{\lambda_N},
\\ f_i(b) & = f_i\left( b^{\prime} \otimes b^{(N)} \right) \otimes
u_{\lambda_N},
\\ \varepsilon_i(b) & = \max\bigl( \varepsilon_i(b^{\prime}) -
\varphi_i\left( b^{(N)} \right), 0 \bigr),
\\ \varphi_i(b) & = \varphi_i(b^{\prime}) + \max\left(
\varphi_i\left( b^{(N)} \right) - \varepsilon_i(b^{\prime}), 0 \right),
\end{aligned}

where b^{\prime} = b^{(0)} \otimes \cdots \otimes b^{(N-1)}. To translate this into a finite list, we consider a finite sequence b^{(0)} \otimes \cdots \otimes b^{(N-1)} \otimes b^{(N)}_{\lambda_N} and if

f_i\left( b^{(0)} \otimes \cdots b^{(N-1)} \otimes
b^{(N)}_{\lambda_N} \right) = b_0 \otimes \cdots \otimes b^{(N-1)}
\otimes f_i\left( b^{(N)}_{\lambda_N} \right),

then we take the image as b^{(0)} \otimes \cdots \otimes f_i\left(
b^{(N)}_{\lambda_N}\right) \otimes b^{(N+1)}_{\lambda_{N+1}}. Similarly we remove b^{(N)}_{\lambda_{N}} if we have b_0 \otimes \cdots
\otimes b^{(N-1)} \otimes b^{(N-1)}_{\lambda_{N-1}} \otimes
b^{(N)}_{\lambda_N}. Additionally if

e_i\left( b^{(0)} \otimes \cdots \otimes b^{(N-1)} \otimes
b^{(N)}_{\lambda_N} \right) = b^{(0)} \otimes \cdots \otimes
b^{(N-1)} \otimes e_i\left( b^{(N)}_{\lambda_N} \right),

then we consider this to be 0.

REFERENCES:

[HK02]Introduction to Quantum Groups and Crystal Bases. Jin Hong and Seok-Jin Kang. 2002. Volume 42. Graduate Studies in Mathematics. American Mathematical Society.

INPUT:

  • B – A single or list of U_q^{\prime} perfect crystal(s) of level l
  • weight – A weight in \overline{P}_l^+

EXAMPLES:

sage: B = crystals.KirillovReshetikhin(['A',2,1], 1,1)
sage: L = RootSystem(['A',2,1]).weight_space()
sage: C = crystals.KyotoPathModel(B, L.fundamental_weight(0))
sage: mg = C.module_generators[0]; mg
[[[3]]]
sage: mg.f_string([0,1,2,2])
[[[3]], [[3]], [[1]]]

An example of type A_5^{(2)}:

sage: B = crystals.KirillovReshetikhin(['A',5,2], 1,1)
sage: L = RootSystem(['A',5,2]).weight_space()
sage: C = crystals.KyotoPathModel(B, L.fundamental_weight(0))
sage: mg = C.module_generators[0]; mg
[[[-1]]]
sage: mg.f_string([0,2,1,3])
[[[-3]], [[2]], [[-1]]]
sage: mg.f_string([0,2,3,1])
[[[-3]], [[2]], [[-1]]]

An example of type D_3^{(2)}:

sage: B = crystals.KirillovReshetikhin(['D',3,2], 1,1)
sage: L = RootSystem(['D',3,2]).weight_space()
sage: C = crystals.KyotoPathModel(B, L.fundamental_weight(0))
sage: mg = C.module_generators[0]; mg
[[]]
sage: mg.f_string([0,1,2,0])
[[[0]], [[1]], []]

An example using multiple crystals of the same level:

sage: B1 = crystals.KirillovReshetikhin(['A',2,1], 1,1)
sage: B2 = crystals.KirillovReshetikhin(['A',2,1], 2,1)
sage: L = RootSystem(['A',2,1]).weight_space()
sage: C = crystals.KyotoPathModel([B1, B2, B1], L.fundamental_weight(0))
sage: mg = C.module_generators[0]; mg
[[[3]]]
sage: mg.f_string([0,1,2,2])
[[[3]], [[1], [3]], [[3]]]
sage: mg.f_string([0,1,2,2,2])
sage: mg.f_string([0,1,2,2,1,0])
[[[3]], [[2], [3]], [[1]], [[2]]]
sage: mg.f_string([0,1,2,2,1,0,0,2])
[[[3]], [[1], [2]], [[1]], [[3]], [[1], [3]]]
class Element(parent, list)

Bases: sage.combinat.crystals.tensor_product.TensorProductOfRegularCrystalsElement

An element in the Kyoto path model.

e(i)

Return the action of e_i on self.

EXAMPLES:

sage: B = crystals.KirillovReshetikhin(['A',2,1], 1,1)
sage: L = RootSystem(['A',2,1]).weight_space()
sage: C = crystals.KyotoPathModel(B, L.fundamental_weight(0))
sage: mg = C.module_generators[0]
sage: all(mg.e(i) is None for i in C.index_set())
True
sage: mg.f(0).e(0) == mg
True
epsilon(i)

Return \varepsilon_i of self.

EXAMPLES:

sage: B = crystals.KirillovReshetikhin(['A',2,1], 1,1)
sage: L = RootSystem(['A',2,1]).weight_space()
sage: C = crystals.KyotoPathModel(B, L.fundamental_weight(0))
sage: mg = C.module_generators[0]
sage: [mg.epsilon(i) for i in C.index_set()]
[0, 0, 0]
sage: elt = mg.f(0)
sage: [elt.epsilon(i) for i in C.index_set()]
[1, 0, 0]
sage: elt = mg.f_string([0,1,2])
sage: [elt.epsilon(i) for i in C.index_set()]
[0, 0, 1]
sage: elt = mg.f_string([0,1,2,2])
sage: [elt.epsilon(i) for i in C.index_set()]
[0, 0, 2]
f(i)

Return the action of f_i on self.

EXAMPLES:

sage: B = crystals.KirillovReshetikhin(['A',2,1], 1,1)
sage: L = RootSystem(['A',2,1]).weight_space()
sage: C = crystals.KyotoPathModel(B, L.fundamental_weight(0))
sage: mg = C.module_generators[0]
sage: mg.f(2)
sage: mg.f(0)
[[[1]], [[2]]]
sage: mg.f_string([0,1,2])
[[[2]], [[3]], [[1]]]
phi(i)

Return \varphi_i of self.

EXAMPLES:

sage: B = crystals.KirillovReshetikhin(['A',2,1], 1,1)
sage: L = RootSystem(['A',2,1]).weight_space()
sage: C = crystals.KyotoPathModel(B, L.fundamental_weight(0))
sage: mg = C.module_generators[0]
sage: [mg.phi(i) for i in C.index_set()]
[1, 0, 0]
sage: elt = mg.f(0)
sage: [elt.phi(i) for i in C.index_set()]
[0, 1, 1]
sage: elt = mg.f_string([0,1])
sage: [elt.phi(i) for i in C.index_set()]
[0, 0, 2]

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