Hyperbolicity and fundamental groups of complex quasi-projective varieties (III): applications

TL;DR

This paper introduces the concept of h-special varieties, proves their fundamental groups have virtually nilpotent linear representations, and addresses conjectures related to the structure and properties of complex quasi-projective varieties.

Contribution

It defines h-special varieties, proves their fundamental groups have virtually nilpotent images under linear representations, and provides examples and structure theorems addressing longstanding conjectures.

Findings

Fundamental groups of h-special varieties have virtually nilpotent linear images.

Established a structure theorem for varieties with big, semisimple fundamental group representations.

Provided examples illustrating properties of non-compact, quasi-projective varieties.

Abstract

This paper is Part III of a series of three. We begin by introducing the notion of $h$-special varieties, which can be seen as varieties "chain-connected by the Zariski closures of entire curves." We prove that if $X$ is either a special complex quasi-projective variety in the sense of Campana or an $h$-special variety, then for any linear representation $\varrho:\pi_1(X)\to \mathrm{GL}_N(\mathbb{C})$, the image $\varrho(\pi_1(X))$ is virtually nilpotent. We also provide examples showing that this result is sharp, leading to a revised form of Campana's abelianity conjecture for smooth quasi-projective varieties. In addition, we prove a structure theorem for quasi-projective varieties with big and semisimple representations of the fundamental groups, thereby addressing a conjecture by Koll\'ar in 1995. We also construct several examples of quasi-projective varieties that are special and $h$-special, highlighting certain atypical properties of the non-compact case in contrast with the projective setting.