Numerical characterization of the hard Lefschetz classes of dimension two

Jiajun Hu, Jian Xiao
Mathematics, Algebraic Geometry, Algebraic Geometry (math.AG), Combinatorics (math.CO)
2023-09-09 16:00:00
We study the numerical characterization of two dimensional hard Lefschetz classes given by the complete intersections of nef classes. In Shenfeld and van Handel's breakthrough work on the characterization of the extremals of the Alexandrov-Fenchel inequality for convex polytopes, they proposed an open question on the algebraic analogue of the characterization. By taking further inspiration from our previous work with Shang on hard Lefschetz theorems for free line bundles, we formulate and refine the conjectural picture more precisely and settle the open question when the collection of nef classes is given by a rearrangement of supercriticality, which in particular includes the big nef collection as a special case. The main results enable us to refine some previous results and study the extremals of Hodge index inequality, and more importantly provide the first series of examples of hard Lefschetz classes of dimension two both in algebraic geometry and analytic geometry, in which one can allow nontrivial augmented base locus and thus drop the semi-ampleness or semi-positivity assumption. As a key ingredient of the numerical characterization, we establish a local Hodge index inequality for Lorentzian polynomials, which is the algebraic analogue of the local Alexandrov-Fenchel inequality obtained by Shenfeld-van Handel for convex polytopes. This result holds in broad contexts, e.g., it holds on a smooth projective variety, on a compact K\"ahler manifold and on a Lorentzian fan, which contains the Bergman fan of a matroid or polymatroid as a typical example.
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