I have asked this question here (*), but there are no answer. 

Let $n \in \mathbb N^*$, $\{a_0,...,a_n\} \subset \mathbb R_*^+$. We suppose $Eq : \sum\limits_{k=0}^n a_k f^k(x)=0$ have no linear solution.

Determinate the solution of $Eq$ for $f \in C^\infty (\mathbb R)$.

PS : $f^2(x)=f \circ f (x)$ and $f^0(x)=x$

(*) : https://artofproblemsolving.com/community/c6h3164353_functional_equation