Components and singularities of Quot schemes and varieties of commuting matrices

Abstract

We investigate the variety of commuting matrices. We classify its components for any number of matrices of size at most 7. We prove that starting from quadruples of size 8 matrices, this scheme has generically nonreduced components, while up to degree 7 it is generically reduced. Our approach is to recast the problem as deformations of modules and generalize an array of methods: apolarity, duality and Białynicki-Birula decompositions to this setup. We include a thorough review of our methods to make the paper self-contained and accessible to both algebraic and linear-algebraic communities. Our results give the corresponding statements for the Quot schemes of points, in particular we classify the components of $Quo{t}_d(O_{{\mathbb{A}}^n}^{\oplus r})$ for $d\le 7$ and all $r$, $n$.

arXiv commentv4: final, minor corrections v3: minor v2: light corrections to initial submission. Comments still welcome v1: Comments most welcome! Initial submission: slight changes probably will occur. 52 pages