Large Deviations for the d'Arcais Numbers

TL;DR

This paper establishes a large deviation principle for coefficients of the d'Arcais polynomials, linking their asymptotic behavior to a Legendre-Fenchel transform and the abundancy index.

Contribution

It proves a Bahadur-Rao type large deviation formula for the coefficients of d'Arcais polynomials, connecting asymptotics to a specific rate function.

Findings

The coefficients satisfy a large deviation principle as n→∞.

The rate function is the Legendre-Fenchel transform of a function related to the q-Pochhammer symbol.

The results relate to the abundancy index in number theory.

Abstract

The d'Arcais polynomials $P_n(z)$ for $n\in\{0,1,\dots\}$ are defined as $\sum_{n=0}^{\infty} P_n(z) q^n = \exp(-z\ln((q;q)_{\infty}))$ where the $q$-Pochhammer symbol is $(q;q)_{\infty} = \prod_{k=1}^{\infty} (1-q^k)$ for $|q|<1$. Denoting the coefficients for $n \in \mathbb{N}$ by the formula $P_n(z) = \sum_{k=1}^{n} A(2,n,k) z^k/n!$, we prove that $k_n! A(2,n,k_n)/n!$ satisfies a Bahadur-Rao type large deviation formula in the limit $n \to \infty$ with $k_n/n \to \kappa \in [0,1)$ as long as $k_n \to \infty$. The large deviation rate function is the Legendre-Fenchel transform $g^*(-\kappa)$ where $g(\kappa) = f^{-1}(\kappa)$ for the function $f : (0,\infty) \to \mathbb{R}$ given by $f(y)= \ln(-\ln((e^{-y};e^{-y})_{\infty}))$. We relate this fact to information about the abundancy index.