Structure of a sequence with prescribed zero-sum subsequences: Rank Two $p$-groups

Abstract

Let $G=(\mathbb Z/n\mathbb Z) \oplus (\mathbb Z/n\mathbb Z)$. Let $\mathsf {s}_{\leq k}(G)$ be the smallest integer $\ell$ such that every sequence of $\ell$ terms from $G$, with repetition allowed, has a nonempty zero-sum subsequence with length at most $k$. It is known that $\mathsf {s}_{\leq 2n-1-k}(G)=2n-1+k$ for $k\in [0,n-1]$, with the structure of extremal sequences showing this bound tight determined when $k\in \{0,1,n-1\}$, and for various special cases when $k\in [2,n-2]$. For the remaining values $k\in [2,n-2]$, the characterization of extremal sequences of length $2n-2+k$ avoiding a nonempty zero-sum of length at most $2n-1-k$ remained open in general, with it conjectured that they must all have the form $e_1^{[n-1]} \boldsymbol{\cdot} e_2^{[n-1]} \boldsymbol{\cdot} (e_1 +e_2)^{[k]}$ for some basis $(e_1,e_2)$ for $G$. Here $x^{[n]}$ denotes a sequence consisting of the term $x$ repeated $n$ times. In this paper, we establish this conjecture for all $k\in [2,n-2]$ when $n$ is prime, which in view of other recent work, implies the conjectured structure for all rank two abelian groups.