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arXiv · 1806.07180

Positivity of the CM line bundle for families of K-stable klt Fano varieties

Giulio Codogni·Zsolt Patakfalvi
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Abstract

The Chow-Mumford (CM) line bundle is a functorial line bundle on the base of any family of klt Fano varieties. It is conjectured that it yields a polarization on the moduli space of K-poly-stable klt Fano varieties. Proving ampleness of the CM line bundle boils down to showing semi-positivity/positivity statements about the CM-line bundle for families with K-semi-stable/K-polystable fibers. We prove the necessary semi-positivity statements in the K-semi-stable situation, and the necessary positivity statements in the uniform K-stable situation, including in both cases variants assuming K-stability only for general fibers. Our statements work in the most general singular situation (klt singularities), and the proofs are algebraic, except the computation of the limit of a sequence of real numbers via the central limit theorem of probability theory. We also present an application to the classification of Fano varieties. Additionally, our semi-positivity statements work in general for log-Fano pairs.

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Giulio Codogni, Zsolt Patakfalvi. 2018-06-19. Positivity of the CM line bundle for families of K-stable klt Fano varieties. https://arxiv.org/abs/1806.07180

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