2-braid groups and positivity phenomenons in Hecke and Temperley-Lieb algebras
Presenter
February 15, 2022
Abstract
There is a well-known homomorphism from Artin's braid group to (the
group of invertible elements of the) Iwahori-Hecke algebra of the
symmetric group, or more generally from any Artin-Tits group to the
corresponding Hecke algebra. Consider the positive lifts of the elements of the
Coxeter group in the Artin-Tits group. Then their images in the Hecke
algebra yield the so-called standard basis of the Hecke algebra. Elements of the standard
basis have a positive expansion in one of Kazhdan and Lusztig's
canonical bases, i.e., have coefficients which are Laurent polynomials with nonnegative coefficients.
In the case where the Coxeter group is finite, the
positive lifts of the elements of the Coxeter group in the Artin-Tits group are the
so-called simple elements of the classical Garside structure. An
alternative Garside structure, called dual Garside structure, was
introduced for spherical type Artin-Tits groups. One can wonder if the
images of these elements in the Hecke algebra still have a positive KL
expansion or not. This is especially interesting in type A, as simple dual
braids yield a basis of the Temperley-Lieb quotient of the Hecke
algebra.
We will explain how positivity of images of simple dual braids can be
obtained in spherical type using a generalization of Kazhdan and
Lusztig's inverse positivity, which predicts that certain elements of
Artin-Tits groups, which we call ""Mikado braids"", have a positive
Kazhdan-Lustig expansion, together with the fact that simple dual braids
are Mikado braids. The positivity of the KL expansion of Mikado braids,
shown for finite Weyl groups by Dyer and Lehrer, can be generalized to
arbitrary Coxeter systems by adapting a result of Elias and Williamson
on the perversity of minimal Rouquier complexes of positive simple braids to a
""twisted"" setting as introduced by Dyer, and asks the question of
determining which braids have a minimal braid complex which is perverse.