Regularity and Koszul property of symbolic powers of monomial ideals

Abstract

Let $I$ be a homogeneous ideal in a polynomial ring over a field. Let $I^{(n)}$ be the $n$-th symbolic power of $I$. Motivated by results about ordinary powers of $I$, we study the asymptotic behavior of the regularity function $\text{reg}(I^{(n)})$ and the maximal generating degree function $\omega (I^{(n)})$, when $I$ is a monomial ideal. It is known that both functions are eventually quasi-linear. We show that, in addition, the sequences $\{\text{reg}{I}^{(n)}/n{\}}_n$ and $\{\omega (I^{(n)})/n{\}}_n$ converge to the same limit, which can be described combinatorially. We construct an example of an equidimensional, height two squarefree monomial ideal $I$ for which $\omega (I^{(n)})$ and $\text{reg}(I^{(n)})$ are not eventually linear functions. For the last goal, we introduce a new method for establishing the componentwise linearity of ideals. This method allows us to identify a new class of monomial ideals whose symbolic powers are componentwise linear.

36 pages. New results and examples added (see 3.10, 4.7-4.11), and minor corrections made throughout. The notation for the maximal generating degree changed from $d(I)$ to $\omega (I)$. To appear in Math. Z