Homology rings of affine grassmannians and positively multiplicative graphs

Abstract

Let $\mathfrak{g}$ be an untwisted affine Lie algebra with associated Weyl group $W_a$. To any level 0 weight $γ$ we associate a weighted graph $Γ_γ$ that encodes the orbit of $γ$ under the action $W_a$. We show that the graph $Γ_γ$ encodes the periodic orientation of certain subsets of alcoves in $W_a$ and therefore can be interpreted as an automaton determining the reduced expressions in these subsets. Then, by using some relevant quotients of the homology ring of affine Grassmannians, we show that $Γ_γ$ is positively multiplicative. This allows us in particular to compute the structure constants of the homology rings using elementary linear algebra on multiplicative graphs. In another direction, the positivity of $Γ_γ$ yields the key ingredients to study a large class of central random walks on alcoves.