Algebraic reverse Khovanskii--Teissier inequality via Okounkov bodies

TL;DR

This paper proves an algebraic version of the reverse Khovanskii--Teissier inequality for nef divisors on projective varieties using Okounkov bodies, extending previous analytic results to an algebraic setting.

Contribution

It introduces a purely algebraic proof of the inequality via Okounkov bodies, broadening the scope of applications beyond the analytic setting.

Findings

Established algebraic reverse Khovanskii--Teissier inequality for nef divisors.

Connected inequality to Be9zout-type and degree inequalities.

Extended previous analytic results to algebraic geometry context.

Abstract

Let $X$ be a projective variety of dimension $n$ over an algebraically closed field of arbitrary characteristic and let $A, B, C$ be nef divisors on $X$. We show that for any integer $1\leq k\leq n-1$, $$ (B^k\cdot A^{n-k})\cdot (A^k\cdot C^{n-k})\geq \frac{k!(n-k)!}{n!}(A^n)\cdot (B^k\cdot C^{n-k}). $$ The same inequality in the analytic setting was obtained by Lehmann and Xiao for compact K\"ahler manifolds using the Calabi--Yau theorem, while our approach is purely algebraic using (multipoint) Okounkov bodies. We also discuss applications of this inequality to B\'ezout-type inequalities and inequalities on degrees of dominant rational self-maps.