Iterated sumsets and Hilbert functions

Abstract

Let A be a finite subset of an abelian group (G, +). Let h $\ge$ 2 be an integer. If |A| $\ge$ 2 and the cardinality |hA| of the h-fold iterated sumset hA = A + $\times$ $\times$ $\times$ + A is known, what can one say about |(h -- 1)A| and |(h + 1)A|? It is known that |(h -- 1)A| $\ge$ |hA| (h--1)/h , a consequence of Pl{\"u}nnecke's inequality. Here we improve this bound with a new approach. Namely, we model the sequence |hA| h$\ge$0 with the Hilbert function of a standard graded algebra. We then apply Macaulay's 1927 theorem on the growth of Hilbert functions, and more specifically a recent condensed version of it. Our bound implies |(h -- 1)A| $\ge$ $\theta$(x, h) |hA| (h--1)/h for some factor $\theta$(x, h) > 1, where x is a real number closely linked to |hA|. Moreover, we show that $\theta$(x, h) asymptotically tends to e $\approx$ 2.718 as |A| grows and h lies in a suitable range varying with |A|.