Asymptotic Log-concavity of Dominant Lower Bruhat Intervals via Brunn--Minkowski Inequality

Abstract

Bj\"orner and Ekedahl [Ann. of Math. (2), 170.2(2009), pp. 799--817] pioneered the study of length-counting sequences associated with parabolic lower Bruhat intervals in crystallographic Coxeter groups. In this paper, we study the asymptotic behavior of these sequences in affine Weyl groups. Let $W$ be an affine Weyl group with corresponding Weyl group $W_f$ and ${}^{f}{W}$ be the set of minimal representatives for the right cosets $W_f \backslash W$. Let $t_{\lambda}$ be the translation by a dominant coroot lattice element $\lambda$ and ${}^{f}{b}_i^{t_{\lambda}}$ be the number of elements of length $i$ below $t_\lambda$ in the Bruhat order on ${}^{f}{W}$. We show that the sequence $({}^{f}{b}_i^{t_{\lambda}})_i$ is ''asymptotically log-concave'' in the following sense: The sequence of discrete measures $(\mathfrak{m}_k)_k$ constructed from the $k$-fold dilated sequence $({}^{f}{b}_i^{t_{k\lambda}})_i$, as $k$ tends to infinity, converges weakly to a continuous measure obtained from a polytope $P^\lambda$. Moreover, the sequence of step functions $(S_k)_k$ of $({}^{f}{b}_i^{t_{k\lambda}})_i$ converges uniformly to the density function of this continuous measure. By Brunn--Minkowski inequality, this density is log-concave.