Polynomial values with integer coefficients of the generating functions for Fibonacci polynomials

TL;DR

This paper extends the study of Fibonacci generating functions to polynomial sequences satisfying a specific recurrence, analyzing when these functions take integer values at particular points related to Fibonacci numbers and their polynomial generalizations.

Contribution

It introduces polynomial sequences with a recurrence relation and characterizes when their generating functions are integer-valued at specific algebraic points, generalizing previous Fibonacci number results.

Findings

Generated functions take integer values at specific polynomial points

Results extend to polynomial sequences with recurrence relations

Application to sequences evaluated at square roots of natural numbers

Abstract

Fibonacci polynomials are generalizations of Fibonacci numbers, so it is natural to consider polynomial versions of the various results for Fibonacci numbers. According to Hong, Pongsriiam, Bulawa, and Lee, the generating function of the Fibonacci sequence in the domain of rational numbers, $f(t)=t/(1-t-t^2)$, takes an integer value if and only if $t=F_{k}/F_{k+1}$ for some $k \in \N$ or $t=-F_{k+1}/F_{k}$ for some $k \in \N^{+}$, where $F_{k}$ is the $k$th Fibonacci number. This study is built upon their work by considering polynomial sequences that satisfy the recurrence relation $F_{i+2}(x)=axF_{i+1}(x)+bF_{i}(x)$ with initial values $(F_{0}(x), F_{1}(x))=(0, 1)$, where $a$ and $b$ are positive integers such that $b|a$. As an application, for a square-free natural number $d \in \N$, we verify the results are of the same form as the above for the generating function of the sequence satisfying the recurrence relation $F_{i+2}(\sqrt{d})=a\sqrt{d} F_{i+1}(\sqrt{d})+bF_{i}(\sqrt{d})$ with initial values $(F_{0}(\sqrt{d}), F_{1}(\sqrt{d}))=(0, 1)$.