The saturation problem for refined Littlewood-Richardson coefficients

Abstract: Given partitions $\lambda,\mu,\nu$ with at most $n$ nonzero parts and a permutation $w \in S_n$, the refined Littlewood-Richardson coefficient $c_{\lambda \mu}^\nu(w)$ is the multiplicity of the irreducible $GL_n \mathbb(C)$ module $V(\nu)$ in the so-called Kostant-Kumar submodule $K(\lambda, w, \mu)$ of the tensor product $V(\lambda) \otimes V(\mu)$. We derive a hive model for these coefficients and prove that the saturation property holds if $w$ is $312$-avoiding, $231$-avoiding or a commuting product of such elements. This generalizes the classical Knutson-Tao saturation theorem.