Can the coefficients of a given term in this family of power series have a common divisor? Let $g_m(b)$ be the coefficient of $x^m$ the power series
$$\dfrac{1}{1-x-bx^2}$$
(When $b=1$, this is just the generating function of Fibonacci numbers.) Of course, $g_m(b)$ depends on both $m$ and $b$. Now fix $m$, and let $b$ run from 1 to $\infty$. Can the $g_m(b)$'s have a non-trivial common divisor for some $m$?
It seems reasonable to believe this cannot happen, but I don't see how to prove it. The relation among the $g_m(b)$'s for a fixed $b$ is simple. But for a fixed $m$, I don't see how to relate the $g_m(b)$'s.
 A: No. Note that $g_m$ has constant term $1$. If all values of $g_m(b)$ have a common divisor $d$, then $d | g_m(d)$, but $g_m(d) \equiv g_m(0) = 1 \bmod d$. 

Initially I wanted to resolve this problem using the following lemma, which is unnecessary. But I thought I would keep it here in case anyone was interested.
Lemma: Let $f(x)$ be a polynomial which attains integer values when $x$ is an integer. If $d$ is an integer such that $d | f(x)$ for all positive integers $x$, then $d | f(x)$ for all integers $x$.
Proof. We need to observe the following facts about the finite difference operator $\Delta f(x) = f(x + 1) - f(x)$. 


*

*$d | f(x)$ for all positive integers $x$ if and only if $d | f(1)$ and $d | \Delta f(x)$ for all positive integers $x$.

*$d | f(x)$ for all integers $x$ if and only if $d | f(1)$ and $d | \Delta f(x)$ for all integers $x$. 

*If $f$ is a polynomial, then $\Delta f$ is a polynomial of degree $\deg f - 1$.
It follows that $d | f(x)$ for all positive integers $x$ if and only if
$$d | f(1), \Delta f(1), \Delta^2 f(1), ... \Delta^{\deg f} f(1)$$
and $d | \Delta^{\deg f+1} f(x)$ for all positive integers $x$. But $\Delta^{\deg f+1} f$ is identically zero, hence $d | \Delta^{\deg f+1} f(x)$ for all integers $x$, hence $d | f(x)$ for all integers $x$. 

Incidentally, these polynomials are closely related to the Chebyshev polynomials of the second kind. 
