Weight lattices and weight spaces¶
-
sage.combinat.root_system.weight_space.
WeightSpace
¶ INPUT:
root_system
– a root systembase_ring
– a ring \(R\)extended
– a boolean (default: False)
The weight space (or lattice if
base_ring
is \(\ZZ\)) of a root system is the formal free module \(\bigoplus_i R \Lambda_i\) generated by the fundamental weights \((\Lambda_i)_{i\in I}\) of the root system.This class is also used for coweight spaces (or lattices).
See also
RootSystem()
RootSystem.weight_lattice()
andRootSystem.weight_space()
WeightLatticeRealizations()
EXAMPLES:
sage: Q = RootSystem(['A', 3]).weight_lattice(); Q Weight lattice of the Root system of type ['A', 3] sage: Q.simple_roots() Finite family {1: 2*Lambda[1] - Lambda[2], 2: -Lambda[1] + 2*Lambda[2] - Lambda[3], 3: -Lambda[2] + 2*Lambda[3]} sage: Q = RootSystem(['A', 3, 1]).weight_lattice(); Q Weight lattice of the Root system of type ['A', 3, 1] sage: Q.simple_roots() Finite family {0: 2*Lambda[0] - Lambda[1] - Lambda[3], 1: -Lambda[0] + 2*Lambda[1] - Lambda[2], 2: -Lambda[1] + 2*Lambda[2] - Lambda[3], 3: -Lambda[0] - Lambda[2] + 2*Lambda[3]}
For infinite types, the Cartan matrix is singular, and therefore the embedding of the root lattice is not faithful:
sage: sum(Q.simple_roots()) 0
In particular, the null root is zero:
sage: Q.null_root() 0
This can be compensated by extending the basis of the weight space and slightly deforming the simple roots to make them linearly independent, without affecting the scalar product with the coroots. This feature is currently only implemented for affine types. In that case, if
extended
is set, then the basis of the weight space is extended by an element \(\delta\):sage: Q = RootSystem(['A', 3, 1]).weight_lattice(extended = True); Q Extended weight lattice of the Root system of type ['A', 3, 1] sage: Q.basis().keys() {0, 1, 2, 3, 'delta'}
And the simple root \(\alpha_0\) associated to the special node is deformed as follows:
sage: Q.simple_roots() Finite family {0: 2*Lambda[0] - Lambda[1] - Lambda[3] + delta, 1: -Lambda[0] + 2*Lambda[1] - Lambda[2], 2: -Lambda[1] + 2*Lambda[2] - Lambda[3], 3: -Lambda[0] - Lambda[2] + 2*Lambda[3]}
Now, the null root is nonzero:
sage: Q.null_root() delta
Warning
By a slight notational abuse, the extra basis element used to extend the fundamental weights is called
\delta
in the current implementation. However, in the literature,\delta
usually denotes instead the null root. Most of the time, those two objects coincide, but not for type \(BC\) (aka. \(A_{2n}^{(2)}\)). Therefore we currently have:sage: Q = RootSystem(["A",4,2]).weight_lattice(extended=True) sage: Q.simple_root(0) 2*Lambda[0] - Lambda[1] + delta sage: Q.null_root() 2*delta
whereas, with the standard notations from the literature, one would expect to get respectively \(2\Lambda_0 -\Lambda_1 +1/2 \delta\) and \(\delta\).
Other than this notational glitch, the implementation remains correct for type \(BC\).
The notations may get improved in a subsequent version, which might require changing the index of the extra basis element. To guarantee backward compatibility in code not included in Sage, it is recommended to use the following idiom to get that index:
sage: F = Q.basis_extension(); F Finite family {'delta': delta} sage: index = F.keys()[0]; index 'delta'
Then, for example, the coefficient of an element of the extended weight lattice on that basis element can be recovered with:
sage: Q.null_root()[index] 2
-
class
sage.combinat.root_system.weight_space.
WeightSpaceElement
¶ Bases:
sage.modules.with_basis.indexed_element.IndexedFreeModuleElement
-
is_dominant
()¶ Checks whether an element in the weight space lies in the positive cone spanned by the basis elements (fundamental weights).
EXAMPLES:
sage: W = RootSystem(['A',3]).weight_space() sage: Lambda = W.basis() sage: w = Lambda[1]+Lambda[3] sage: w.is_dominant() True sage: w = Lambda[1]-Lambda[2] sage: w.is_dominant() False
In the extended affine weight lattice, ‘delta’ is orthogonal to the positive coroots, so adding or subtracting it should not effect dominance
sage: P = RootSystem(['A',2,1]).weight_lattice(extended=true) sage: Lambda = P.fundamental_weights() sage: delta = P.null_root() sage: w = Lambda[1]-delta sage: w.is_dominant() True
-
scalar
(lambdacheck)¶ The canonical scalar product between the weight lattice and the coroot lattice.
Todo
- merge with_apply_multi_module_morphism
- allow for any root space / lattice
- define properly the return type (depends on the base rings of the two spaces)
- make this robust for extended weight lattices (\(i\) might be “delta”)
EXAMPLES:
sage: L = RootSystem(["C",4,1]).weight_lattice() sage: Lambda = L.fundamental_weights() sage: alphacheck = L.simple_coroots() sage: Lambda[1].scalar(alphacheck[1]) 1 sage: Lambda[1].scalar(alphacheck[2]) 0
The fundamental weights and the simple coroots are dual bases:
sage: matrix([ [ Lambda[i].scalar(alphacheck[j]) ....: for i in L.index_set() ] ....: for j in L.index_set() ]) [1 0 0 0 0] [0 1 0 0 0] [0 0 1 0 0] [0 0 0 1 0] [0 0 0 0 1]
Note that the scalar product is not yet implemented between the weight space and the coweight space; in any cases, that won’t be the job of this method:
sage: R = RootSystem(["A",3]) sage: alpha = R.weight_space().roots() sage: alphacheck = R.coweight_space().roots() sage: alpha[1].scalar(alphacheck[1]) Traceback (most recent call last): ... ValueError: -Lambdacheck[1] + 2*Lambdacheck[2] - Lambdacheck[3] is not in the coroot space
-
to_ambient
()¶ Maps
self
to the ambient space.EXAMPLES:
sage: mu = CartanType(['B',2]).root_system().weight_lattice().an_element(); mu 2*Lambda[1] + 2*Lambda[2] sage: mu.to_ambient() (3, 1)
Warning
Only implemented in finite Cartan type. Does not work for coweight lattices because there is no implemented map from the coweight lattice to the ambient space.
-
to_weight_space
()¶ Map
self
to the weight space.Since \(self.parent()\) is the weight space, this map just returns
self
. This overrides the generic method in \(WeightSpaceRealizations\).EXAMPLES:
sage: mu = CartanType(['A',2]).root_system().weight_lattice().an_element(); mu 2*Lambda[1] + 2*Lambda[2] sage: mu.to_weight_space() 2*Lambda[1] + 2*Lambda[2]
-