Kähler groups and Geometric Group Theory

Thesis

In this thesis we study Kähler groups and their connections to Geometric Group Theory. This work presents substantial progress on three central questions in the field:

1. Which subgroups of direct products of surface groups are Kähler?
2. Which Kähler groups admit a classifying space with finite (n-1)-skeleton but no classifying space with finitely many n-cells?
3. Is it possible to give explicit finite presentations for any of the groups constructed in response to Question 2?

Question 1 was raised by Delzant and Gromov.

Question 2 is intimately related to Question 1: the non-trivial examples of Kähler subgroups of direct products of surface groups never admit a classifying space with finite skeleton.

The only known source of non-trivial examples for Questions 1 and 2 are fundamental groups of fibres of holomorphic maps from a direct product of closed surfaces onto an elliptic curve; the first such construction is due to Dimca, Papadima and Suciu.

Question 3 was posed by Suciu in the context of these examples.

In this thesis we:

• provide the first constraints on Kähler subdirect products of surface groups (Theorem 7.3.1);
• develop new construction methods for Kähler groups from maps onto higher-dimensional complex tori (Section 6.1);
• apply these methods to obtain irreducible examples of Kähler subgroups of direct products of surface groups which arise from maps onto higher-dimensional tori and use them to show that our conditions in Theorem 7.3.1 are minimal (Theorem A);
• apply our construction methods to produce irreducible examples of Kähler groups that (i) have a classifying space with finite (n-1)-skeleton but no classifying space with finite n-skeleton and (ii) do not have a subgroup of finite index which embeds in a direct product of surface groups (Theorem 8.3.1);
• provide a new proof of Biswas, Mj and Pancholi's generalisation of Dimca, Papadima and Suciu's construction to more general maps onto elliptic curves (Theorem 4.3.2) and introduce invariants that distinguish many of the groups obtained from this construction (Theorem 4.6.2); and
• construct explicit finite presentations for Dimca, Papadima and Suciu's groups thereby answering Question 3 (Theorem 5.4.4)).

Authors: Isenrich, C

• DOI: 10.5287/ora-erpbapr1v
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Geometric Group Theory; Geometry and Topology; Complex geometry; Kähler groups
• Subjects: Mathematics