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Take for example $f(z)=e^z$, so the inverse is $z(f) = \ln(f) + n\pi i$ for an arbitrarily chosen (but fixed) branch $n\in\mathbb N$. Now if $f$ is restricted to e.g. $0<|1-f|<1$ such that the essential singularity at $f=0$ is not part of the definition region, is $z(f)$ then analytical in that region?

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    @joriki: oh, I missed that line, thanks! so the answer is simply yes, could you post that as an answer so this question doesn't remain unanswered (to the system I mean)?2011-03-10

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See the Lagrange inversion theorem: "$g$ is analytic at the point $b=f(a)$".