Generalized complexity of surfaces

TL;DR

This paper introduces a generalized complexity invariant for surfaces and Calabi-Yau pairs, showing it characterizes toric varieties and relates to classical theorems, with applications to singularities and local geometry.

Contribution

It defines the generalized complexity for Calabi-Yau pairs and proves its role in characterizing toric surfaces and threefold singularities, extending prior results.

Findings

Generalized complexity 0 implies the surface is toric.

Calabi-Yau surfaces with complexity 0 are either projective plane or product of projective lines.

3-fold singularities with complexity 0 are toric.

Abstract

In this article, we introduce the generalized complexity of a generalized Calabi--Yau pair $(X,B,\textbf{M})$. This invariant compares the dimension of $X$ and Picard rank of $X$ with the sum of the coefficients of $B$ and $\textbf{M}$. It generalizes the complexity introduced by Shokurov. We show that a generalized log Calabi-Yau pair $(X,B,\textbf{M})$ of dimension $2$ with generalized complexity $0$ satisfies that $X$ is toric. This generalizes a result due to Brown, McKernan, Svaldi, and Zhong in the case of surfaces. Furthermore, we show that a generalized klt log Calabi-Yau surface $(X,\textbf{M})$ with generalized complexity $0$ satisfies that $X\simeq \mathbb{P}^2$ or $X\simeq \mathbb{P}^1\times \mathbb{P}^1$. Thus, this invariant interpolates between the characterization of toric varieties and the Kobayashi-Ochiai Theorem. As an application, we show that $3$-fold singularities with generalized complexity $0$ are toric. Furthermore, we show a local version of Kobayashi-Ochiai Theorem in dimension $3$.