Freiman homomorphisms on sparse random sets

Abstract

A result of Fiz Pontiveros shows that if $A$ is a random subset of $\mathbb{Z}_N$ where each element is chosen independently with probability $N^{-1/2+o(1)}$, then with high probability every Freiman homomorphism defined on $A$ can be extended to a Freiman homomorphism on the whole of $\mathbb{Z}_N$. In this paper we improve the bound to $CN^{-2/3}(\log N)^{1/3}$, which is best possible up to the constant factor.Research supported by a Royal Society University Research Fellowship and by ERC Starting Grant 676632. Research supported by a Royal Society 2010 Anniversary Research Professorship