Regularity of Symbolic Powers of Polymatroidal Ideals and Supermodular Prime Intersections

Abstract

Let $S=\mathbb{K}[x_1,\ldots,x_n]$ be a polynomial ring over a field $\mathbb{K}$. For a polymatroidal ideal $I\subseteq S$ generated in degree $d$, we prove $$\omega(I^{(s)})=\operatorname{reg} I^{(s)}=\operatorname{reg} I^s=sd\qquad(s\ge1),$$ where symbolic powers use all associated primes and $\omega$ denotes the largest degree of a minimal generator. Thus every symbolic power has a degree resolution. The argument uses a regularity bound for supermodular prime intersections, obtained from Takayama's formula and a homological estimate for noncover complexes. For every nonzero proper squarefree monomial ideal $L$, we also show $\operatorname{reg} L^{(s)}\le s\min\{m,\lambda(L)-\operatorname{ht}L+1\}$, where $m$ counts the minimal primes and $\lambda(L)$ is the support size, and obtain equality for skeleton and pure skeleton ideals. Finally, for every $d\ge3$, we construct a simple graphic matroid whose basis ideal is generated in degree $d$ and has a second symbolic power that is not componentwise linear.