The Generalised Mukai Conjecture for spherical varieties

Abstract: Associated to every spherical variety $X$, is a rational number $\tilde{\wp}(X)$ which may be computed purely combinatorially. When $X$ is complete, we prove that $\tilde{\wp}(X)\ge0$, with equality if and only if $X$ is isomorphic to a toric variety, if and only if $\tilde{\wp}(X)<1$. This yields a proof of the generalised Mukai conjecture for spherical varieties, as well as a proof of a combinatorial smoothness criterion for spherical varieties, conjectured by Gagliardi and Hofscheier, whereby smoothness along each $G$-orbit of a spherical variety can be checked by solving a linear program defined purely by spherical combinatorial data.