Here it's cited:
the existence of the holomorphic function ${\displaystyle \varphi }$ such that ${\displaystyle \varphi (\varphi (u))=\exp(u)}$ had been demonstrated in 1950 by Hellmuth Kneser.
However I can't find the definition of the function $\varphi(u)$ anywhere.
Does this function truly exist? And if so, What is the function?