Past exam of the geometry course of the University of Oxford / 2008 / B / 8 / iii / Solution

Choose an actual pole $p_i$ of $f$; one exists by nonconstancy and compactness. In a local coordinate $z$ centered there, its simple pole has expansion $f(z)=a_{-1}/z+O(1)$ with $a_{-1}\ne 0$. HenceThe holomorphic inverse function theorem makes $1/f$ a local coordinate. Here the source notation $f^{-1}$ denotes the reciprocal, rather than an inverse map of the meromorphic function.

The chosen pole is among the fixed points, so $\phi$ maps a sufficiently small neighborhood of it into the coordinate neighborhood. If $f=f\circ \phi$, then $w=w\circ \phi$ there. Injectivity of this coordinate forces $\phi (x)=x$ throughout a nonempty open set. The stated identity principle then gives $\phi$ equal to the identity everywhere.