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Motivated by my ignorance here, if $X$ is a projective toric variety, is $$H^m(X, \mathcal O_X) \cong \begin{cases} 0 & m > 0 \\ \mathbb C & m = 1 \end{cases} $$ as for $\mathbb P^n$?

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I take it $H^m$ means dimension of m-th cohomology? –  Matt Dec 2 '10 at 21:25
    
Whoops, I fixed it. –  James Davidoff Dec 2 '10 at 23:11

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up vote 6 down vote accepted

Yes, this is true, at least for varieties over the complex numbers $\mathbb{C}$. Indeed, a toric variety over an algebraically closed field is rational (i.e., birational to projective space). In characteristic zero, rational connectedness is a birational invariant, so toric varieties are rationally connected. Finally, any rationally connected variety is $\mathcal{O}$-acyclic, which is the name for the conclusion that you want. See e.g. here for this last implication.

The conclusion might well hold more generally; I am not an expert in these matters. You may want to ask your question on MathOverflow if you are not satisfied with this answer.

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It's true in finite characteristic as well. See section 3.5 in Fulton's "Toric Varieties" book for how to compute cohomology of line bundles on toric varieties over any field. I don't think he singles out the case of the trivial line bundle; but it is easy to plug it in and see that you get 0. –  David Speyer Apr 28 '11 at 19:39

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