Rigid resolutions and big Betti numbers

Abstract

In the first part of the paper we answer (positively) a question raised by the first author which has to do with some sort of rigity of the tail of resolution of an ideal. Let $I$ be a homogeneous ideal in a polynomial ring over a field of characteristic 0. Denote by $β_i(I)$ the $i$-th Betti number of $I$ and by $Gin(I)$ the revlex generic initial ideal of $I$. In general one has $β_i(I)\leq β_i(Gin(I))$ and we show that if $β_i(I)=β_i(Gin(I))$ for some $i$ then $β_j(I)=β_j(Gin(I))$ for all $j>i$. In the second part of the paper we answer a question of Eisenbud and Huneke. We prove that if $I$ is $m$-primary and $I\subset m^d$ then $β_i(m^d)\leq β_i(Gin(I))$ for all $i$.