A positivity property in the based ring of the lowest two-sided cell

TL;DR

This paper explores positivity properties in the based ring of the lowest two-sided cell of an affine Weyl group, providing formulas and a new positive basis under certain conditions.

Contribution

It introduces formulas for coefficients in the based ring related to the Langlands dual group and defines a new positive basis for specific subrings.

Findings

Formulas for coefficients in the based ring involving generalized exponents.

A new positive basis for the subring of the asymptotic Hecke algebra.

Results applicable to the canonical left cell and partial results for GL_n.

Abstract

Let $W_{\mathrm{aff}}$ be an extended affine Weyl group and $\mathbf{H}$ and $J$ be the corresponding affine and asymptotic Hecke algebras with standard bases $\{T_x\}$ and $\{t_w\}$, respectively. Viewing $J$ as a subalgebra of the $\mathbf{q}^{-\frac{1}{2}}$-adic completion of $\mathbf{H}$, we give formulas for the coefficient of $T_x$ in $t_w$ for various $x$ and $w$ in the lowest two-sided cell, in terms of generalized exponents of the Langlands dual group, under a hypothesis on the left cell containing $w$. In particular our results hold for the canonical left cell. For such $w$ we also define a seemingly new positive basis for the corresponding subring of $J$. For $\mathrm{GL}_n$, we give partial results for some other cells.