Is there an analog of Kodaira vanishing for singular varieties - MathOverflow

Is there an analog of Kodaira vanishing for singular varieties

2011-10-14T13:18:19Z

I would like to know what kind of analogs of Kodaira vanishing theorem are valid for singular varieties. For example, is the following true: let $X$ be a projective Gorenstein variety and let $\omega_X$ be its canonical bundle. Is it true that $H^i(L\otimes \omega_X)=0$ for $i>0$ for an ample line bundle $L$?

Answer by J.C. Ottem for Is there an analog of Kodaira vanishing for singular varieties

2011-10-14T13:33:29Z

No. The following counterexample is due to Sommese:

Let $Y$ be the projective bundle $\pi:\mathbb{P}(O\oplus O(1)^{\oplus 3})\to \mathbb{P}^1$. Let $M$ be the tautological bundle on $Y$ and take a general member $X\in|M\otimes \pi^*O(-1)^{\oplus 4})|$. Then $X$ is a normal, projective, Gorenstein 3-fold. If $L$ is the line budle $M\otimes \pi^*O(1)$, one can also check that $H^1(X,O(K_X+L))=\mathbb{C}$.

However, it is known that the Kodaira vanishing theorem holds if $X$ has log canonical singularities. There are also weaker versions in the theorem in the paper 'D. Arapura and D. B. Jaffe On Kodaira Vanishing for Singular Varieties Proc. A.M.S, 105, No. 4, pp. 911-916, 1989.'

Answer by Karl Schwede for Is there an analog of Kodaira vanishing for singular varieties

2011-10-14T15:50:21Z

Indeed, as JC Ottem points out, Kodaira holds for log canonical (even semi-log canonical singularities). There's also a way to quickly deduce that Kodaira vanishing holds for Du Bois singularities (either from the Ambro-Fujino machinary or mimicking arguments of Kollar, let me know if you want details, perhaps I should put it on mathoverflow since it's not written down anywhere).

However, I should probably point out that it's totally trivial to see that Kodaira vanishing holds for rational singularities. Here's the proof:

Let $\pi : Y \to X$ be a resolution. Note $R \pi_* O_Y \cong O_X$ and so $R \pi O_Y(-\pi^* L) \cong O_X(-L)$ for any line bundle $L$. Fix $L$ to be ample. By a spectral sequence/composition of derived functors argument:

$$H^i(X, O_X(-L)) = H^i(Y, O_Y(-\pi^* L)).$$

But $\pi^* L$ is nef and big and the vanishing of the right hand side is just Kawamata-Viehweg vanishing and Serre duality.