Symbolic powers of polymatroidal ideals
Antonino Ficarra, Somayeh Moradi
TL;DR
The paper investigates symbolic powers of polymatroidal ideals, focusing on componentwise linearity and Castelnuovo-Mumford regularity. It develops general bounds via the symbolic Rees algebra and introduces the $x$-condition to guarantee linear quotients for all symbolic powers. The authors verify the central conjectures for key families (squarefree Veronese, matching-matroidal of Veronese type, and various non-squarefree classes) and establish the Conforti-Cornuéjols packing-type results for matroidal ideals, along with a broad classification of cases where ordinary and symbolic powers coincide. Collectively, the work advances understanding of when symbolic powers retain linear resolutions and componentwise linearity, linking these properties to structural decompositions such as disjoint supports and products of prime ideals.
Abstract
In this paper, we investigate the componentwise linearity and the Castelnuovo-Mumford regularity of symbolic powers of polymatroidal ideals. For a polymatroidal ideal $I$, we conjecture that every symbolic power $I^{(k)}$ is componentwise linear and $$ \text{reg}\,I^{(k)}=\text{reg}\,I^k $$ for all $k \ge 1$. We prove that $\text{reg}\,I^{(k)}\ge\text{reg}\,I^k$ for all $k \ge 1$ when $I$ has no embedded associated primes, for instance if $I$ is a matroidal ideal. Moreover, we establish a criterion on the symbolic Rees algebra $\mathcal{R}_s(I)$ of a monomial ideal of minimal intersection type which guarantees that every symbolic power $I^{(k)}$ has linear quotients and, hence, is componentwise linear for all $k\ge1$. By applying our criterion to squarefree Veronese ideals and certain matching-matroidal ideals, we verify both conjectures for these families. We establish the Conforti-Cornuéjols conjecture for any matroidal ideal, and we show that a matroidal ideal is packed if and only if it is the product of monomial prime ideals with pairwise disjoint supports. Furthermore, we identify several classes of non-squarefree polymatroidal ideals for which the ordinary and symbolic powers coincide. Hence, we confirm our conjectures for transversal polymatroidal ideals and principal Borel ideals. Finally, we verify our conjectures for all polymatroidal ideals either generated in small degrees or in a small number of variables.