Let $F \in \mathbb{C}(X)$ be a rational function with complex coefficients (and $\mathbb{C}(X)$ the set of such functions) i.e. $F=\frac P Q$ for $P,Q \in \mathbb{C}[X]$
I need to show that
$F(X)=F(\frac 1 X) \iff \exists G \in \mathbb{C}(X), G(X+ \frac 1 X)=F$
Showing $\Leftarrow$ is easy but I don't know how to show $\Rightarrow$.
Can someone help me please ?
Thanks in advance.