Hyperbolicity and fundamental groups of complex quasi-projective varieties
Benoit Cadorel, Ya Deng, Katsutoshi Yamanoi
TL;DR
This work establishes deep links between hyperbolicity properties and fundamental groups of complex quasi-projective varieties via linear representations of π1. It develops a unified framework combining non-abelian Hodge theory, Nevanlinna theory, and geometric group theory to derive pseudo Picard hyperbolicity, a generalized Green-Griffiths-Lang conjecture, and a refined Campana-type abelianity picture, including non-Archimedean reduction via spectral covers. The results include a factorisation mechanism for representations, a structure theorem for varieties with reductive big representations, and a spectral-cover approach that propagates positivity to obtain log general type in broad settings. Collectively, the paper advances the understanding of how representation-theoretic data constrain the global geometry and hyperbolicity of quasi-projective varieties, with notable implications for Shafarevich-type problems and the algebraic structure of fundamental groups.
Abstract
This paper investigates the relationship between the hyperbolicity of complex quasi-projective varieties $X$ and the (topological) fundamental group $π_1(X)$ in the presence of a linear representation $\varrho: π_1(X) \to {\rm GL}_N(\mathbb{C})$. We present our main results in three parts. Firstly, we show that if $\varrho$ is bigand the Zariski closure of $\varrho(π_1(X))$ semisimple, then for any $X^σ:=X\times_σ\mathbb{C}$ where $σ\in {\rm Aut}(\mathbb{C}/\mathbb{Q})$, there exists a proper Zariski closed subset $Z \subsetneqq X^σ$ such that any closed irreducible subvariety $V$ of $X^σ$ not contained in $Z$ is of log general type, and any holomorphic map from the punctured disk $\mathbb{D}^*$ to $X^σ$ with image not contained in $Z$ does not have an essential singularity at the origin. In particular, all entire curves in $X^σ$ lie on $Z$. We provide examples to illustrate the optimality of this condition. Secondly, assuming that $\varrho$ is big and reductive, we prove the generalized Green-Griffiths-Lang conjecture for $X^σ$. Furthermore, if $\varrho$ is large, we show that the special subsets of $X^σ$ that capture the non-hyperbolicity locus of $X^σ$ from different perspectives are equal, and this subset is proper if and only if $X$ is of log general type. Lastly, we prove that if $X$ is a special quasi-projective manifold in the sense of Campana or $h$-special, then $\varrho(π_1(X))$ is virtually nilpotent. We provides examples to demonstrate that this result is sharp and thus revise Campana's abelianity conjecture for smooth quasi-projective varieties. To prove these theorems, we develop new features in non-abelian Hodge theory, geometric group theory, and Nevanlinna theory. Some byproducts are obtained.