Stabilization of Boij-Söderberg Decompositions of Ideal Powers

Abstract

Given an ideal $I$ we investigate the decompositions of Betti diagrams of the graded family of ideals $\{I^k \}_k$ formed by taking powers of $I$. We prove conjectures of Engström and show that there is a stabilization in the Boij-Söderberg decompositions of $I^k$ for $k>>0$ when $I$ is a homogeneous ideal with generators in a single degree. In particular, the number of terms in the decompositions with positive coefficients remains constant for $k>>0$, the pure diagrams appearing in each decomposition have the same shape, and the coefficients of these diagrams are given by polynomials in $k$. We also show that a similar result holds for decompositions with arbitrary coefficients arising from other chains of pure diagrams.