Counting subalgebras of $\mathfrak{o}^n$

TL;DR

This paper introduces a new method to analyze the enumeration of finite-index subalgebras of $rak{o}^n$, providing explicit formulas and bounds for their zeta functions, with applications to counting irreducible subrings.

Contribution

The paper develops a novel approach to study cotype zeta functions of subalgebras, expressing them as sums of integrals and computing them explicitly in several cases.

Findings

Derived a finite sum expression for the zeta function as $rak{o}$-adic integrals.

Established a lower bound for the abscissa of convergence of the subalgebra zeta function.

Provided explicit formulas for the cotype zeta function of subalgebras of $rak{o}^4$.

Abstract

Let $\mathfrak{o}$ be a compact discrete valuation ring and $n\geq 2$. We introduce a method to study the cotype zeta function of subalgebras of $\mathfrak{o}^n$. This multivariable series encodes the number of finite-index subalgebras $\Lambda$ of the $\mathfrak{o}$-algebra $\mathfrak{o}^n$ of a given elementary divisor type. We express this zeta function as a finite sum of $\mathfrak{o}$-adic integrals and compute these integrals in many cases. As a first application, we recover known results in a natural way from our approach. For instance, we obtain a lower bound for the abscissa of convergence of the subalgebra zeta function of $\mathfrak{o}^n$ by exhibiting an explicit pole. We also determine the number of irreducible subrings of $\mathfrak{o}^n$ of small index. As a second application, we give an explicit formula for the cotype zeta function of subalgebras of $\mathfrak{o}^4$.