The generalized Mukai conjecture for symmetric varieties

Abstract

We associate to any complete spherical variety $X$ a certain nonnegative rational number $\wp (X)$, which we conjecture to satisfy the inequality $\wp (X)\le \dim X-\mathrm{rank}X$ with equality holding if and only if $X$ is isomorphic to a toric variety. We show that, for spherical varieties, our conjecture implies the generalized Mukai conjecture on the pseudo-index of smooth Fano varieties due to Bonavero, Casagrande, Debarre, and Druel. We also deduce from our conjecture a smoothness criterion for spherical varieties. It follows from the work of Pasquier that our conjecture holds for horospherical varieties. We are able to prove our conjecture for symmetric varieties.

33 pages, 2 figures, 6 tables

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