On the inverse of a Fibonacci number modulo a Fibonacci number being a Fibonacci number

Abstract

Let $(F_n)_{n \geq 1}$ be the sequence of Fibonacci numbers. For all integers $a$ and $b \geq 1$ with $\gcd(a, b) = 1$, let $[a^{-1} \!\bmod b]$ be the multiplicative inverse of $a$ modulo $b$, which we pick in the usual set of representatives $\{0, 1, \dots, b-1\}$. Put also $[a^{-1} \!\bmod b] := \infty$ when $\gcd(a, b) > 1$. We determine all positive integers $m$ and $n$ such that $[F_m^{-1} \bmod F_n]$ is a Fibonacci number. This extends a previous result of Prempreesuk, Noppakaew, and Pongsriiam, who considered the special case $m \in \{3, n - 3, n - 2, n - 1\}$ and $n \geq 7$. Let $(L_n)_{n \geq 1}$ be the sequence of Lucas numbers. We also determine all positive integers $m$ and $n$ such that $[L_m^{-1} \bmod L_n]$ is a Lucas number.