The M\"obius function on Affine Grassmannian elements

Abstract

To any saturated chain in the affine Weyl group whose translation parts are sufficiently regular, we associate a near path and a far path in the quantum Bruhat graph. Using this, working in the Bruhat order on the minimal-length representatives of the cosets in the affine Weyl group with respect to the finite Weyl group, we characterize the pairs of elements for which the M\"obius function is nonzero. This is applied to obtain explicit expansions in the $K$-theory of affine Grassmannians, of the basis of ideal sheaves into the basis of structure sheaves of Schubert varieties.