Quadratic Gorenstein rings and the Koszul property I

Abstract

Let $R$ be a standard graded Gorenstein algebra over a field presented by quadrics. Conca, Rossi, and Valla have shown that such a ring is Koszul if $\mathrm{reg}R\le 2$ or if $\mathrm{reg}=3$ and $c=\mathrm{codim}R\le 4$, and they ask whether this is true for $\mathrm{reg}R=3$ in general. We determine sufficient conditions on a non-Koszul quadratic Cohen-Macaulay ring $R$ that guarantee the Nagata idealization $\tilde{R}=R⋉\ddot{I}{‰}_R(-a-1)$ is a non-Koszul quadratic Gorenstein ring. We use this to negatively answer the question of Conca-Rossi-Valla, constructing non-Koszul quadratic Gorenstein rings of regularity 3 for all $c\ge 9$.