Grassmannians in the Lattice points of Dilations of the Standard Simplex

Abstract

A remarkable connection between the cohomology ring ${\rm H^{\ast}(Gr}(d, d+r),\Z)$ of the Grasssmannian ${\rm Gr}(d,d+r)$ and the lattice points of the dilation $r\Delta_{d}$ of the standard d-simplex is investigated. The natural grading on the cohomology induces different gradings of the lattice points of $r\Delta_{d}$. This leads to different refinements of the Ehrhart polynomial $L_{\Delta_{d}}(r)$ of the standard $d$-simplex. We study two of these refinements which are defined by the weights $(1,1,\dots,1)$ and $(1,2,\dots, d)$. One of the refinements interprets the Poincar\'e polynomial ${\rm P(Gr}(d,d+r),z)$ as the counting of the lattice points which lie on the slicing hyperplanes of the dilation $r\Delta_d$. Therefore, on the combinatorial level the Poincar\'e polynomial of the Grassmannian Gr$(d,d+r)$ is a refinement of the Ehrhart polynomial $L_{\Delta_d}(r)$ of the standard $d$-simplex $\Delta_{d}$.