On curves lying on a rational normal surface scroll

TL;DR

This paper investigates the minimal free resolutions of non-ACM divisors on rational normal surface scrolls, revealing a decomposition of Betti tables under certain conditions and providing explicit Betti number descriptions.

Contribution

It introduces a decomposition method for Betti tables of divisors on rational normal scrolls when a specific inequality holds, and fully describes Betti numbers for particular scroll cases.

Findings

Betti table decomposition for divisors with $a_2 \\geq 2a_1 -1$

Complete Betti number descriptions for $S(1,r-2)$ and $S(2,r-3)$ cases

Conditions under which Betti tables can be expressed as sums of simpler tables

Abstract

In this paper, we study the minimal free resolution of non-ACM divisors $X$ of a smooth rational normal surface scroll $S=S(a_1 ,a_2 ) \subset \mathbb{P}^r$. Our main result shows that for $a_2 \geq 2a_1 -1$, there exists a nice decomposition of the Betti table of $X$ as a sum of much simpler Betti tables. As a by-product of our results, we obtain a complete description of the graded Betti numbers of $X$ for the cases where $S=S(1,r-2)$ for some $r \geq 3$ and $S=S(2,r-3)$ for some $r \geq 6$.