The largest $(k, \ell)$-sum-free sets in compact abelian groups

Abstract

A subset $A$ of a finite abelian group is called $(k,\ell)$-sum-free if $kA \cap \ell A=\emptyset.$ In this paper, we extend this concept to compact abelian groups and study the question of how large a measurable $(k,\ell)$-sum-free set can be. For integers $1 \leq k <\ell$ and a compact abelian group $G$, let $$\lambda_{k,\ell}(G)=\sup\{ \mu(A): kA \cap \ell A =\emptyset \}$$ be the maximum possible size of a $(k,\ell)$-sum-free subset of $G$. We prove that if $G=\mathbb{I} \times M$, where $\mathbb{I}$ is the identity component of $G$, then $$\lambda_{k, \ell}(G)=\max \left\{ \lambda_{k, \ell}(M), \lambda_{k, \ell}(\mathbb{I}) \right\}.$$ Moreover, if $\mathbb{I}$ is nontrivial, then $\lambda_{k,\ell}(\mathbb{I})=\frac{1}{k+\ell}$. Finally, we discuss how this problem motivates a new framework for studying $(k,\ell)$-sum-free sets in finite groups.