This question already has an answer here:
Based on this question: How to calculate $f(x)$ in $f(f(x)) = e^x$? I would like to know if I can get a function such that $f:\mathbb R \to \mathbb R^+$, defined by $f\circ f(x)=e^x$. My guess is no, but I can't prove it, I need help.
Note that different than the previous question, the function is from $\mathbb R$ to $\mathbb R^+$.
Thanks a lot