Generalized Hilbert Functions

Abstract

Let $M$ be a finite module and let $I$ be an arbitrary ideal over a Noetherian local ring. We define the generalized Hilbert function of $I$ on $M$ using the 0th local cohomology functor. We show that our definition re-conciliates with that of Ciuperc$\breve{{\rm a}}$. By generalizing Singh's formula (which holds in the case of $\lambda(M/IM)<\infty$), we prove that the generalized Hilbert coefficients $j_0,..., j_{d-2}$ are preserved under a general hyperplane section, where $d={\rm dim}\,M$. We also keep track of the behavior of $j_{d-1}$. Then we apply these results to study the generalized Hilbert function for ideals that have minimal $j$-multiplicity or almost minimal $j$-multiplicity. We provide counterexamples to show that the generalized Hilbert series of ideals having minimal or almost minimal $j$-multiplicity does not have the `expected' shape described in the case where $\lambda(M/IM)<\infty$. Finally we give a sufficient condition such that the generalized Hilbert series has the desired shape.