For a positive integer $a$, define $F_1 ^{(a)}=1$, $F_2 ^{(a)}=a$ and for $n>2$, $F_n ^{(a)}=F_{n-1} ^{(a)}+F_{n-2} ^{(a)}$. A positive integer is fibonatic when it is equal to $F_n ^{(a)}$ for a positive integer $a$ and $n>3$. Prove that there are infintely many not fibonatic integers.
Problem
Source: Brazilian National Olympiad 2020 2 Level 3
Tags: Fibonacci, number theory, Brazilian Math Olympiad, Brazil, Analytic Number Theory
16.03.2021 03:06
21.03.2021 07:13
17.06.2023 02:29
Here's a (not completely sound) argument that's still insightful The fibonatic integers are of the form $aF_n+F_{n-1}$, so we just need the number to not be $F_{n-1}\pmod{F_n}$. Heuristically, the density of these numbers is $$\prod_{n\ge 3}\left(1-\frac{1}{F_n}\right)$$Because the "probability" for a large number to be $F_{n-1}\pmod{F_n}$ is $\frac{1}{F_n}$. To show this is positive, we can say $1-\frac{1}{F_n}>1-C^n$ where $0<C<\frac{1}{\phi}$, for sufficiently large $n$. So it's enough to show $P=\prod_{n\ge 3}\left(1-C^n\right)>0$. For a small enough $C$, it is always true that $\ln (1-C^n)>-\frac{1}{C^n}$. Then $\ln P>\sum_{n\ge 3}-\frac{1}{C^n}>-\infty$ and hence $P>0$.