Talk:Riemann–Hurwitz formula

Examples needed

This page should include examples

• counting the genus of hyperelliptic curves
• counting the genus of some superelliptic curves (e.g. y^3 = f(x))
• relate these to schemes in P^1\times P^1
• Also mention wild ramification, https://math.berkeley.edu/~mcivor/256B.pdf — Preceding unsigned comment added by Wundzer (talk • contribs) 18:22, 29 April 2020 (UTC)

A possible mistake in the formula $\chi (S')-r=N\cdot (\chi (S)-b)$

It seems to me that the formula $\chi (S')-r=N\cdot (\chi (S)-b)$ and its proof are wrong as stated, since the fiber of a branch point may contain unramified points, and thus the restriction of $\pi$ to the unramified points as stated in the proof is undefined. Any univariate polynomial whose derivative has more than one distinct root provides a counterexample when considered as a map of Riemann spheres.

To correct the error, it seems sufficient to replace $r$ by the size of the union of the fibers of the branch points, which contains all the ramification points together with perhaps some unramified points, denote this quantity by $b'$. In this formulation, the proof works as stated in the article, and we can also deduce it from the usual formulation of Riemann-Hurwitz, as we have $N\cdot b-b'=\sum _{P\in S'}(e_{P}-1)$ since for any $Q\in S$ we have $N=\sum _{P\in \pi ^{-1}(Q)}e_{P}$ Horrific Necktie (talk) 09:36, 12 August 2022 (UTC)