DOCUMENTA MATHEMATICA, Vol. 15 (2010), 957-972

Jason Starr and Johan de Jong

Almost Proper GIT-Stacks and Discriminant Avoidance

We prove that the classifying stack of an reductive group scheme over a field is very close to being proper. Using this we prove a result about isotrivial families of varieties. Fix a polarized variety with reductive automorphism group. To prove that every isotrivial family with this fibre has a rational section it suffices to prove this when the base is projective, i.e., the discriminant of the family is empty.

2010 Mathematics Subject Classification:

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