On the least common multiple of shifted powers

TL;DR

This paper investigates the asymptotic growth of the logarithm of the least common multiple of shifted powers for periodic and random sequences, revealing quadratic growth rates with explicit constants.

Contribution

It establishes effective formulas for the growth of LCMs of shifted powers for both periodic and random sequences, including explicit constants and probabilistic results.

Findings

Log LCM grows quadratically with n for periodic sequences.

Explicit constant C_s depends on the sequence's periodic pattern.

For random sequences, the growth rate involves the dilogarithm function.

Abstract

Let $a \geq 2$ be an integer. We prove that for every periodic sequence $(s_n)_{n \geq 1}$ in $\{-1, +1\}$ there exists an effectively computable rational number $C_\mathbf{s} > 0$ such that \begin{equation*} \log\operatorname{lcm}(a + s_1, a^2 + s_2, \dots, a^n + s_n) \sim C_\mathbf{s} \cdot \frac{\log a}{\pi^2} \cdot n^2 , \end{equation*} as $n \to +\infty$, where $\operatorname{lcm}$ denotes the least common multiple. Furthermore, we show that if $(s_n)_{n \geq 1}$ is a sequence of independent and uniformly distributed random variables in $\{-1, +1\}$ then \begin{equation*} \log\operatorname{lcm}(a + s_1, a^2 + s_2, \dots, a^n + s_n) \sim 6 \operatorname{Li}_2\!\big(\tfrac1{2}\big) \cdot \frac{\log a}{\pi^2} \cdot n^2 , \end{equation*} with probability $1 - o(1)$, as $n \to +\infty$, where $\operatorname{Li}_2$ is the dilogarithm function.