Cauchy-Davenport type inequalities, I

TL;DR

This paper generalizes and strengthens classical additive number theory inequalities, providing new bounds for sumsets in groups and semigroups, with implications for cyclic groups and abelian groups.

Contribution

It introduces a new inequality for sumsets in groups, extending previous results and unifying several classical theorems in additive combinatorics.

Findings

Established a lower bound for |X+Y| in groups with specific conditions.

Generalized the Chowla-Pillai theorem for finite cyclic groups.

Extended the Hamidoune-Shatrowsky theorem to abelian groups.

Abstract

Let $\mathbb G = (G, +)$ be a group (either abelian or not). Given $X, Y \subseteq G$, we denote by $\langle Y \rangle$ the subsemigroup of $\mathbb G$ generated by $Y$, and we set $$\gamma(Y) := \sup_{y_0 \in Y} \inf_{y_0 \ne y \in Y} {\rm ord}(y - y_0)$$ if $|Y| \ge 2$ and $\gamma(Y) := |Y|$ otherwise. We prove that if $\langle Y \rangle$ is commutative, $Y$ is non-empty, and $X+2Y \neq X + Y + y$ for some $y \in Y$, then $$ |X+Y| \ge |X|+\min(\gamma(Y), |Y| - 1). $$ Actually, this is obtained from a more general result, which improves on previous work of the author on sumsets in cancellative semigroups, and yields a comprehensive generalization, and in some cases a considerable strengthening, of various additive theorems, notably including the Chowla-Pillai theorem (on sumsets in finite cyclic groups) and the specialization to abelian groups of the Hamidoune-Shatrowsky theorem.