Positivity in the shadow of Hodge index theorem

TL;DR

This paper investigates the Hodge index theorem on compact Kähler manifolds, characterizing Lorentzian classes and applying these insights to complex geometry problems like the kernel of Lefschetz operators, Teissier's proportionality, and stability estimates.

Contribution

It introduces a Lorentzian polynomial framework for the Hodge index theorem, extending previous results to broader classes and providing new geometric characterizations and stability estimates.

Findings

Characterization of kernel faces of Lefschetz operators on nef classes.

New approach to Teissier's proportionality problem using hard Lefschetz property.

Stability estimates related to complex Monge-Ampère equations and Newton-Okounkov bodies.

Abstract

Taking a compact K\"{a}hler manifold as playground, we explore the powerfulness of Hodge index theorem. A main object is the Lorentzian classes on a compact K\"{a}hler manifold, behind which the characterization via Lorentzian polynomials over the K\"{a}hler cone and hence the validity of Hodge index theorem. Along the exploration, we discover several applications in complex geometry that may be unexpected before. (1) For a Lefschetz type operator given by the complete intersection of nef classes, we give a complete characterization of its kernel face against the pseudo-effective cone. (2) We provide a new approach to Teissier's proportionality problem from the validity of hard Lefschetz property. This perspective enables us to establish the extremals for the Brunn-Minkowski inequality on a strictly Lorentzian class, and thus also characterize the most extremal case for a log-concavity sequence given by the intersection numbers of two nef classes. These Lorentzian classes include the fundamental classes of smooth projective varieties or compact K\"{a}hler manifolds as typical examples, hence our result extends Boucksom-Favre-Jonsson's and Fu-Xiao's results in respective settings to broader contexts, e.g. certain algebraic cycle classes given by reducible subvarieties. (3) Furthermore, we also strengthen the proportionality characterization by comparing various quantitative deficits and establishing stability estimates. Two quantitative sharper stability estimates with close relation with complex Monge--Amp\`{e}re equations and Newton-Okounkov bodies are also discussed.