Components and singularities of Quot schemes and varieties of commuting matrices

Abstract

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 8 × 8 {8\times 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 Quot d ⁡ ( 𝒪 𝔸 n ⊕ r ) {\operatorname{Quot}_{d}(\mathcal{O}_{\mathbb{A}^{n}}^{\oplus r})} for d ≤ 7 {d\leq 7} and all r, n.