On the arithmetic of the join rings over finite fields

TL;DR

This paper explores the arithmetic properties of join rings over finite fields, including structural decompositions, zeta functions, and connections to primitive roots and special primes like Mersenne and Fermat primes.

Contribution

It introduces a generalized augmentation map, computes zeta functions, and characterizes join rings over finite fields with units of bounded order, revealing links to special primes.

Findings

Constructed a structural decomposition via a generalized augmentation map.

Computed the zeta function of the join of group rings.

Identified connections between units in join rings and Mersenne and Fermat primes.

Abstract

Given a collection $\{ G_i\}_{i=1}^d$ of finite groups and a ring $R$, we have previously introduced and studied certain foundational properties of the join ring $\mathcal{J}_{G_1, G_2, \ldots, G_d}(R)$. This ring bridges two extreme worlds: matrix rings $M_n(R)$ on one end, and group rings $R[G]$ on the other. The construction of this ring was motivated by various problems in graph theory, network theory, nonlinear dynamics, and neuroscience. In this paper, we continue our investigations of this ring, focusing more on its arithmetic properties. We begin by constructing a generalized augmentation map that gives a structural decomposition of this ring. This decomposition allows us to compute the zeta function of the join of group rings. We show that the join of group rings is a natural home for studying the concept of simultaneous primitive roots for a given set of primes. This concept is related to the order of the unit group of the join of group rings. Finally, we characterize the join of group rings over finite fields with the property that the order of every unit divides a fixed number. Remarkably, Mersenne and Fermat primes unexpectedly emerge within the context of this exploration.