The Separating Noether Number of Finite Abelian Groups

TL;DR

This paper exactly determines the separating Noether number for all finite abelian groups using additive combinatorics, avoiding previous assumptions, and confirms a conjecture about the support of extremal separating atoms for groups of rank at least two.

Contribution

It provides a complete formula for the separating Noether number of finite abelian groups and introduces novel combinatorial methods, bypassing previous restrictive assumptions.

Findings

Exact formulas for eta_{ ext{sep}}(G) for all finite abelian groups.

Validation of a conjecture on the support of extremal separating atoms for groups of rank  or more.

Identification of exceptions in the cyclic (rank 1) case.

Abstract

For a finite abelian group $G$, let $\beta_{\mathrm{sep}}(G)$ denote its separating Noether number. We determine $\beta_{\mathrm{sep}}(G)$ exactly for every finite abelian group $ G \cong C_{n_1}\oplus \cdots \oplus C_{n_r}$ with $ 1<n_1 \mid \cdots \mid n_r. $ If $r=2s-1$, then $$ \beta_{\mathrm{sep}}(G)=n_s+n_{s+1}+\cdots+n_r, $$ whereas if $r=2s$, then $$ \beta_{\mathrm{sep}}(G)=\frac{n_s}{p_1}+n_{s+1}+\cdots+n_r, $$ where $p_1$ denotes the smallest prime divisor of $n_1$. Our proof is additive-combinatorial in nature. It avoids the Davenport-equality assumption $\mathsf{D}(n_sG)=\mathsf{D}^{*}(n_sG)$ used in previous works. The key ingredients are a geometric reduction of auxiliary sequences via the novel construction of geodesic surrogates, alongside a uniform lifting procedure for relation groups. As an application, we prove that if $r\ge 2$, then every extremal separating atom $A$ over $G_0$ with $|G_0|\le r+1$ satisfies $|\supp(A)|=|G_0|=r+1$. Equivalently, the conjectured support conclusion of Schefler, Zhao, and Zhong holds for all finite abelian groups of rank at least $2$. By contrast, the rank-$1$ case is exceptional: for cyclic groups, the analogous conjectural conclusion is false.