The syzygies of some thickenings of determinantal varieties

The vector space of $m\times n$ complex matrices ($m\geq n$) admits a natural action of the group $\operatorname {GL}=\operatorname {GL}_m\times \operatorname {GL}_n$ via row and column operations. For positive integers $a,b$, we consider the ideal $I_{a\times b}$ defined as the smallest $\operatorname {GL}$-equivariant ideal containing the $b$-th powers of the $a\times a$ minors of the generic $m\times n$ matrix. We compute the syzygies of the ideals $I_{a\times b}$ for all $a,b$, together with their $\operatorname {GL}$-equivariant structure, generalizing earlier results of Lascoux for the ideals of minors ($b=1$), and of Akin–Buchsbaum–Weyman for the powers of the ideals of maximal minors ($a=n$). Our methods rely on a nice connection between commutative algebra and the representation theory of the superalgebra $\mathfrak {gl}(m|n)$, as well as on our previous calculation of $\operatorname {Ext}$ modules done in the context of describing local cohomology with determinantal support. Our results constitute an important ingredient in the proof by Nagpal–Sam–Snowden of the first non-trivial Noetherianity results for twisted commutative algebras which are not generated in degree one.