# How to solve the iterated function $f(f(x))=x^2+x$? [duplicate]

Any given setting for $f$ is acceptable.

Iterated function

## marked as duplicate by Did, José Carlos Santos, Martin Sleziak, Community♦Aug 26 '17 at 10:21

Apparently I gave a complete solution for this exact problem some years ago, it is one of the links below.

I have the book Kuczma, M., Choczewski B., and Ger, R. (1990). Iterative Functional Equations. Cambridge University Press which was helpful.

The bad news is that the derivative at the fixpoint $x=0$ is $1.$ This means that you can construct Ecalle's solution for $x \geq 0$ so that $f(0) = 0,$ $f'(0) = 1,$ we get $f \in C^\infty,$ and then, for $x > 0,$ we get $x \in C^\omega.$ I should emphasize that there is no holomorphic solution in a neighborhood that includes the origin.

My impression is that we lose smoothness at $x = - \frac{1}{2}.$