# Euler characteristic of Weierstrass divisor [closed]

I have a strong feeling that, for a compact connected Riemann surface $X$ of genus $g>0$, the Euler characteristic of the Weierstrass divisor $W$ equals $$\chi(X,\mathcal{O}_X(W)) = (g-1)^2.$$ Is this true?

By Riemann-Roch, the Euler characteristic is given by $$\chi(X,W) = g^3 -g + 1- g = g^3-2g+1.$$

This is not equal to $(g-1)^2$ for $g>1$.

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## closed as too localized by Angelo, Qiaochu Yuan, Dan Petersen, Felipe Voloch, Mark SapirJan 7 '12 at 19:12

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Just apply Riemann-Roch: the divisor $W$ has degree $g^3 - g$. I voted to close as "Too localized". –  Angelo Jan 7 '12 at 7:19
You're right. I asked this question too quickly. –  Harized Jan 7 '12 at 7:20

$$\sum_{p\in X} w(p) = (g-1)g(g+1).$$
By Riemann-Roch, the Euler characteristic then equals $g^3-2g+1$, which is different from your formula.