## Over which schemes can there exist non-trivial G_a bundles?

The group scheme G_a here is the one-dimensional additive group.

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 You can escape the underscores with a backslash so that they don't cause a problem (i.e. type \_ to get _). Better yet, instead of typing H^1(X,O\_X), you can type H1(X,OX) to get really pretty-looking output. – Anton Geraschenko♦ Oct 28 2009 at 5:16