Kostant--Kumar modules: presentation and multiplicities

Abstract

Kostant--Kumar modules $K(λ,w,μ)$ are submodules of a tensor product $V(λ)\otimes V(μ)$ of irreducible highest weight modules over a symmetrizable Kac--Moody algebra, indexed by Weyl group elements $w$; their decomposition numbers $c^ν_{λμ}(w)$ refine ordinary tensor product multiplicities. We study them module-theoretically. We show that $c^ν_{λμ}(w)$ is computed by a natural quotient of the Kostant--Parthasarathy--Ranga Rao--Varadarajan multiplicity space, via orthogonal projection onto a Demazure module. For $\mathfrak{g}$ finite-dimensional semisimple or symmetric Kac--Moody, we present $K(λ,w,μ)$ by generators and relations, extending the presentation of Demazure modules due to Joseph, Polo and Mathieu. We apply the presentation to obtain upper bounds on $c^ν_{λμ}(w)$ and to study Schur positivity.