On the gradient of a monomial ideal

Abstract

Let $K$ be a field of characteristic zero, let $I\subset S=K[x_1,\dots ,x_n]$ be a homogeneous ideal, and let $\partial (I)$ be its gradient ideal. We study the relationship between $\mathrm{reg}I$ and $\mathrm{reg}\partial (I)$. While earlier work by Busé, Dimca, Schenck, and Sticlaru showed these regularities are generally incomparable for hypersurface ideals, we prove they remain incomparable even for monomial ideals with linear resolution, answering a question of J. Herzog. In fact, for any integers $a\in \mathbb{Z}$ and $b\ge -1$, we construct monomial ideals $I$ and $J$ such that $\mathrm{reg}I-\mathrm{reg}\partial (I)=a$, $\mathrm{reg}\partial (J)-\mathrm{reg}J=b$ and $J$ has linear resolution. We introduce monomial ideals with differential linear resolution as those monomial ideals whose all iterated gradient ideals have linear resolution. We prove that polymatroidal ideals, equigenerated (strongly) stable ideals, powers of edge ideals with linear resolution, complementary edge ideals with linear resolution, and certain equigenerated squarefree monomial ideals with many generators satisfy this property.