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A local Lefschetz theorem

SPEAKER: Bhargav Bhatt

TITLE: A local Lefschetz theorem

ABSTRACT: A classical theorem of Lefschetz asserts that non-trivial line bundles on a smooth projective variety of dimension >= 3 remain non-trivial upon restriction to an ample divisor. In SGA2, Grothendieck recast this result in purely local algebraic terms. Answering a question raised recently by Koll’ar, we will explain how this local reformulation remains true under much milder hypotheses than those in SGA2. Our method uses a vanishing theorem in characteristic p, and formal geometry over certain very large (non-noetherian) schemes. This is joint work with Johan de Jong.