Linear series and existence of branched covers
Abstract
Description
In this paper, we use the perspective of linear series, and in particular results following from the degeneration tools of limit linear series, to give a number of new results on existence and non-existence of branched covers of the projective line in positive characteristic. Our results are both in terms of ramification indices and the sharper invariant of monodromy groups, and the first class of results are obtained by intrinsically algebraic and positive-characteristic arguments.
19 pages. Heavily revised. Non-sharp existence results removed, many examples added, and main theorem stated in stronger form with simpler proof, using the results of math.AG/0609118
19 pages. Heavily revised. Non-sharp existence results removed, many examples added, and main theorem stated in stronger form with simpler proof, using the results of math.AG/0609118