AUTHORS:
Bases: sage.structure.sage_object.SageObject, sage.structure.unique_representation.UniqueRepresentation
A Cartan type realized from a (Dynkin) diagram folding.
Given a Cartan type \(X\), we say \(\hat{X}\) is a folded Cartan type of \(X\) if there exists a diagram folding of the Dynkin diagram of \(\hat{X}\) onto \(X\).
A folding of a simply-laced Dynkin diagram \(D\) with index set \(I\) is an automorphism \(\sigma\) of \(D\) where all nodes any orbit of \(\sigma\) are not connected. The resulting Dynkin diagram \(\hat{D}\) is induced by \(I / \sigma\) where we identify edges in \(\hat{D}\) which are not incident and add a \(k\)-edge if we identify \(k\) incident edges and the arrow is pointing towards the indicent note. We denote the index set of \(\hat{D}\) by \(\hat{I}\), and by abuse of notation, we denote the folding by \(\sigma\).
We also have scaling factors \(\gamma_i\) for \(i \in \hat{I}\) and defined as the unique numbers such that the map \(\Lambda_j \mapsto \gamma_j \sum_{i \in \sigma^{-1}(j)} \Lambda_i\) is the smallest proper embedding of the weight lattice of \(X\) to \(\hat{X}\).
If the Cartan type is simply laced, the default folding is the one induced from the identity map on \(D\).
If \(X\) is affine type, the default embeddings we consider here are:
and were chosen based on virtual crystals. In particular, the diagram foldings extend to crystal morphisms and gives a realization of Kirillov-Reshetikhin crystals for non-simply-laced types as simply-laced types. See [OSShimo03] and [FOS09] for more details. Here we can compute \(\gamma_i = \max(c) / c_i\) where \((c_i)_i\) are the translation factors of the root system. In a more type-dependent way, we can define \(\gamma_i\) as follows:
We note that \(\gamma_i\) only depends upon \(X\).
If the Cartan type is finite, then we consider the classical foldings/embeddings induced by the above affine foldings/embeddings:
For more information on Cartan types, see sage.combinat.root_system.cartan_type.
Other foldings may be constructed by passing in an optional folding_of second argument. See below.
INPUT:
Note
If \(X\) is an affine type, we assume the special node is fixed under \(\sigma\).
EXAMPLES:
sage: fct = CartanType(['C',4,1]).as_folding(); fct
['C', 4, 1] as a folding of ['A', 7, 1]
sage: fct.scaling_factors()
Finite family {0: 2, 1: 1, 2: 1, 3: 1, 4: 2}
sage: fct.folding_orbit()
Finite family {0: (0,), 1: (1, 7), 2: (2, 6), 3: (3, 5), 4: (4,)}
A simply laced Cartan type can be considered as a virtual type of itself:
sage: fct = CartanType(['A',4,1]).as_folding(); fct
['A', 4, 1] as a folding of ['A', 4, 1]
sage: fct.scaling_factors()
Finite family {0: 1, 1: 1, 2: 1, 3: 1, 4: 1}
sage: fct.folding_orbit()
Finite family {0: (0,), 1: (1,), 2: (2,), 3: (3,), 4: (4,)}
Finite types:
sage: fct = CartanType(['C',4]).as_folding(); fct
['C', 4] as a folding of ['A', 7]
sage: fct.scaling_factors()
Finite family {1: 1, 2: 1, 3: 1, 4: 2}
sage: fct.folding_orbit()
Finite family {1: (1, 7), 2: (2, 6), 3: (3, 5), 4: (4,)}
sage: fct = CartanType(['F',4]).dual().as_folding(); fct
['F', 4] relabelled by {1: 4, 2: 3, 3: 2, 4: 1} as a folding of ['E', 6]
sage: fct.scaling_factors()
Finite family {1: 1, 2: 1, 3: 2, 4: 2}
sage: fct.folding_orbit()
Finite family {1: (1, 6), 2: (3, 5), 3: (4,), 4: (2,)}
REFERENCES:
[OSShimo03] | M. Okado, A. Schilling, M. Shimozono. “Virtual crystals and fermionic formulas for type \(D_{n+1}^{(2)}\), \(A_{2n}^{(2)}\), and \(C_n^{(1)}\)”. Representation Theory. 7 (2003). 101-163. doi:10.1.1.192.2095, Arxiv 0810.5067. |
Return the Cartan type of self.
EXAMPLES:
sage: fct = CartanType(['C', 4, 1]).as_folding()
sage: fct.cartan_type()
['C', 4, 1]
Return the Cartan type of the virtual space.
EXAMPLES:
sage: fct = CartanType(['C', 4, 1]).as_folding()
sage: fct.folding_of()
['A', 7, 1]
Return the orbits under the automorphism \(\sigma\) as a dictionary (of tuples).
EXAMPLES:
sage: fct = CartanType(['C', 4, 1]).as_folding()
sage: fct.folding_orbit()
Finite family {0: (0,), 1: (1, 7), 2: (2, 6), 3: (3, 5), 4: (4,)}
Return the scaling factors of self.
EXAMPLES:
sage: fct = CartanType(['C', 4, 1]).as_folding()
sage: fct.scaling_factors()
Finite family {0: 2, 1: 1, 2: 1, 3: 1, 4: 2}
sage: fct = CartanType(['BC', 4, 2]).as_folding()
sage: fct.scaling_factors()
Finite family {0: 1, 1: 1, 2: 1, 3: 1, 4: 2}
sage: fct = CartanType(['BC', 4, 2]).dual().as_folding()
sage: fct.scaling_factors()
Finite family {0: 2, 1: 1, 2: 1, 3: 1, 4: 1}
sage: CartanType(['BC', 4, 2]).relabel({0:4, 1:3, 2:2, 3:1, 4:0}).as_folding().scaling_factors()
Finite family {0: 2, 1: 1, 2: 1, 3: 1, 4: 1}