The spanning number and the independence number of a subset of an abelian group

Abstract

Let $A=\{a_1,a_2,\dots, a_m\}$ be a subset of a finite abelian group $G$. We call $A$ {\it $t$-independent} in $G$, if whenever $$λ_1a_1+λ_2a_2+\cdots +λ_m a_m=0$$ for some integers $λ_1, λ_2, \dots , λ_m$ with $$|λ_1|+|λ_2|+\cdots +|λ_m| \leq t,$$ we have $λ_1=λ_2= \cdots = λ_m=0$, and we say that $A$ is {\it $s$-spanning} in $G$, if every element $g$ of $G$ can be written as $$g=λ_1a_1+λ_2a_2+\cdots +λ_m a_m$$ for some integers $λ_1, λ_2, \dots , λ_m$ with $$|λ_1|+|λ_2|+\cdots +|λ_m| \leq s.$$ In this paper we give an upper bound for the size of a $t$-independent set and a lower bound for the size of an $s$-spanning set in $G$, and determine some cases when this extremal size occurs. We also discuss an interesting connection to spherical combinatorics.