English

On the l.c.m. of shifted Fibonacci numbers

Number Theory 2020-07-28 v1

Abstract

Let (Fn)n1(F_n)_{n \geq 1} be the sequence of Fibonacci numbers. Guy and Matiyasevich proved that \begin{equation*} \log \operatorname{lcm} (F_1, F_2, \dots, F_n) \sim \frac{3 \log \alpha}{\pi^2} \cdot n^2 \quad \text{as } n \to +\infty, \end{equation*} where lcm\operatorname{lcm} is the least common multiple and α:=(1+5)/2\alpha := \big(1 + \sqrt{5}) / 2 is the golden ratio. We prove that for every periodic sequence s=(sn)n1\mathbf{s} = (s_n)_{n \geq 1} in {1,+1}\{-1,+1\} there exists an effectively computable rational number Cs>0C_{\mathbf{s}} > 0 such that \begin{equation*} \log \operatorname{lcm} (F_3 + s_3, F_4 + s_4, \dots, F_n + s_n) \sim \frac{3 \log \alpha}{\pi^2} \cdot C_\mathbf{s} \cdot n^2 , \quad \text{as } n \to +\infty . \end{equation*} Moreover, we show that if (sn)n1(s_n)_{n \geq 1} is a sequence of independent uniformly distributed random variables in {1,+1}\{-1,+1\} then \begin{equation*} \mathbb{E}\big[\log \operatorname{lcm} (F_3 + s_3, F_4 + s_4, \dots, F_n + s_n)\big] \sim \frac{3 \log \alpha}{\pi^2} \cdot \frac{15 \operatorname{Li}_2(1 / 16)}{2} \cdot n^2 , \quad \text{as } n \to +\infty , \end{equation*} where Li2\operatorname{Li}_2 is the dilogarithm function.

Keywords

Cite

@article{arxiv.2007.13330,
  title  = {On the l.c.m. of shifted Fibonacci numbers},
  author = {Carlo Sanna},
  journal= {arXiv preprint arXiv:2007.13330},
  year   = {2020}
}