The Koszul property for spaces of quadrics of codimension three

Abstract

In this paper we prove that, if $\mathbb{k}$ is an algebraically closed field of characteristic different from 2, almost all quadratic standard graded $\mathbb{k}$-algebras $R$ such that $\dim_{\mathbb{k}}R_2 = 3$ are Koszul. More precisely, up to graded $\mathbb{k}$-algebra homomorphisms and trivial fiber extensions, we find out that only two (or three, when the characteristic of $\mathbb{k}$ is 3) algebras of this kind are non-Koszul. Moreover, we show that there exist nontrivial quadratic standard graded $\mathbb{k}$-algebras with $\dim_{\mathbb{k}}R_1 = 4$, $\dim_{\mathbb{k}}R_2 = 3$ that are Koszul but do not admit a Gr\"obner basis of quadrics even after a change of coordinates, thus settling in the negative a question asked by Conca.