# Examples of finite Weyl groups¶

sage.categories.examples.finite_weyl_groups.Example
class sage.categories.examples.finite_weyl_groups.SymmetricGroup(n=4)

An example of finite Weyl group: the symmetric group, with elements in list notation.

The purpose of this class is to provide a minimal template for implementing finite Weyl groups. See SymmetricGroup for a full featured and optimized implementation.

EXAMPLES:

sage: S = FiniteWeylGroups().example()
sage: S
The symmetric group on {0, ..., 3}
sage: S.category()
Category of finite irreducible weyl groups


The elements of this group are permutations of the set $$\{0,\ldots,3\}$$:

sage: S.one()
(0, 1, 2, 3)
sage: S.an_element()
(1, 2, 3, 0)


The group itself is generated by the elementary transpositions:

sage: S.simple_reflections()
Finite family {0: (1, 0, 2, 3), 1: (0, 2, 1, 3), 2: (0, 1, 3, 2)}


Only the following basic operations are implemented:

All the other usual Weyl group operations are inherited from the categories:

sage: S.cardinality()
24
sage: S.long_element()
(3, 2, 1, 0)
sage: S.cayley_graph(side = "left").plot()
Graphics object consisting of 120 graphics primitives


Alternatively, one could have implemented sage.categories.coxeter_groups.CoxeterGroups.ElementMethods.apply_simple_reflection() instead of simple_reflection() and product(). See CoxeterGroups().example().

class Element
has_right_descent(i)

EXAMPLES:

sage: S = FiniteWeylGroups().example()
sage: s = S.simple_reflections()
sage: (s[1] * s[2]).has_descent(2)
True
sage: S._test_has_descent()

cartan_type()

Return the Cartan type of self.

EXAMPLES:

sage: FiniteWeylGroups().example().cartan_type()
['A', 3] relabelled by {1: 0, 2: 1, 3: 2}

degrees()

Return the degrees of self.

EXAMPLES:

sage: W = FiniteWeylGroups().example()
sage: W.degrees()
(2, 3, 4)

index_set()

EXAMPLES:

sage: FiniteWeylGroups().example().index_set()
[0, 1, 2]

one()

Implements Monoids.ParentMethods.one().

EXAMPLES:

sage: FiniteWeylGroups().example().one()
(0, 1, 2, 3)

product(x, y)

Implements Semigroups.ParentMethods.product().

EXAMPLES:

sage: s = FiniteWeylGroups().example().simple_reflections()
sage: s[1] * s[2]
(0, 2, 3, 1)

simple_reflection(i)

Implement CoxeterGroups.ParentMethods.simple_reflection() by returning the transposition $$(i, i+1)$$.

EXAMPLES:

sage: FiniteWeylGroups().example().simple_reflection(2)
(0, 1, 3, 2)