Varieties of reductions for $gl\_n$

Abstract

We study the varieties of reductions associated to the variety of rank one matrices in $\fgl\_n$. These varieties are defined as natural compactifications of the different ways to write the identity matrix as a sum of $n$ rank one matrices. Equivalently, they compactify the quotient of $PGL\_n$ by the normalizer of a maximal torus. In particular, we prove that for $n=4$ we get a 12-dimensional Fano variety with Picard number one, index 3, and canonical singularities.