Dynamics of the Fibonacci Order of Appearance Map

TL;DR

This paper investigates the dynamics of the order of appearance map in Fibonacci numbers, proving the existence of infinitely many integers that reach fixed points in a finite number of steps and providing new constructions and proofs.

Contribution

It establishes the existence of infinitely many integers that reach fixed points in exactly k steps and constructs infinite families of integers for specific fixed points.

Findings

Infinitely many integers iterate to fixed points in exactly k steps.

Constructs infinite families of integers for fixed points of the form 12·5^k.

Provides an alternative proof that all positive integers reach a fixed point after finite iterations.

Abstract

The \textit{order of appearance} $ z(n) $ of a positive integer $ n $ in the Fibonacci sequence is defined as the smallest positive integer $ j $ such that $ n $ divides the $ j $-th Fibonacci number. A \textit{fixed point} arises when, for a positive integer $ n $, we have that the $ n^{\text{th}} $ Fibonacci number is the smallest Fibonacci that $ n $ divides. In other words, $ z(n) = n $. In 2012, Marques proved that fixed points occur only when $ n $ is of the form $ 5^{k} $ or $ 12\cdot5^{k} $ for all non-negative integers $ k $. It immediately follows that there are infinitely many fixed points in the Fibonacci sequence. We prove that there are infinitely many integers that iterate to a fixed point in exactly $ k $ steps. In addition, we construct infinite families of integers that go to each fixed point of the form $12 \cdot 5^{k}$. We conclude by providing an alternate proof that all positive integers $n$ reach a fixed point after a finite number of iterations.