On the monoid generated by a Lucas sequence

TL;DR

This paper investigates the algebraic structure and distribution properties of monoids generated by Lucas sequences, including Fibonacci and Pell numbers, revealing their near-free generation and probabilistic factorization behaviors.

Contribution

It characterizes the monoid generated by Lucas sequences as almost freely generated and provides asymptotic and probabilistic results on their factorization properties.

Findings

Asymptotic formula for counting elements in the monoid

Erdős-Kac type theorems for the number of factors

Gaussian distribution for distinct factors, non-Gaussian when multiplicities are included

Abstract

A Lucas sequence is a sequence of the general form $v_n = (\phi^n - \bar{\phi}^n)/(\phi-\bar{\phi})$, where $\phi$ and $\bar{\phi}$ are real algebraic integers such that $\phi+\bar{\phi}$ and $\phi\bar{\phi}$ are both rational. Famous examples include the Fibonacci numbers, the Pell numbers, and the Mersenne numbers. We study the monoid that is generated by such a sequence; as it turns out, it is almost freely generated. We provide an asymptotic formula for the number of positive integers $\leq x$ in this monoid, and also prove Erd\H{o}s-Kac type theorems for the distribution of the number of factors, with and without multiplicity. While the limiting distribution is Gaussian if only distinct factors are counted, this is no longer the case when multiplicities are taken into account.