On curves lying on a rational normal surface scroll

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 $a_2\ge 2a_1-1$, 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 $r\ge 3$ and $S=S(2,r-3)$ for some $r\ge 6$.

20 pages; final version, to appear in Journal of Pure and Applied Algebra