Ergodic Theoretic Methods in Group Homology

1. Front Matter

   Pages i-ix

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2. Introduction

   • Clara Löh

   Pages 1-4

3. The von Neumann Dimension

   • Clara Löh

   Pages 5-15

4. L2-Betti Numbers

   • Clara Löh

   Pages 17-26

5. The Residually Nite View: Approximation

   • Clara Löh

   Pages 27-35

6. The Dynamical View: Measured Group Theory

   • Clara Löh

   Pages 37-58

7. Invariant Random Subgroups

   • Clara Löh

   Pages 59-71

8. Simplicial Volume

   • Clara Löh

   Pages 73-92

9. Back Matter

   Pages 93-114

   PDF

This book offers a concise introduction to ergodic methods in group homology, with a particular focus on the computation of L²-Betti numbers.

Group homology integrates group actions into homological structure. Coefficients based on probability measure preserving actions combine ergodic theory and homology. An example of such an interaction is provided by L²-Betti numbers: these invariants can be understood in terms of group homology with coefficients related to the group von Neumann algebra, via approximation by finite index subgroups, or via dynamical systems. In this way, L²-Betti numbers lead to orbit/measure equivalence invariants and measured group theory helps to compute L²-Betti numbers. Similar methods apply also to compute the rank gradient/cost of groups as well as the simplicial volume of manifolds.

This book introduces L²-Betti numbers of groups at an elementary level and thendevelops the ergodic point of view, emphasising the connection with approximation phenomena for homological gradient invariants of groups and spaces. The text is an extended version of the lecture notes for a minicourse at the MSRI summer graduate school “Random and arithmetic structures in topology” and thus accessible to the graduate or advanced undergraduate students. Many examples and exercises illustrate the material.

• L2-Betti numbers
• von Neumann dimension
• measured group theory
• approximation of homological invariants
• simplicial volume
• ergodic theory of groups
• homological gradient invariants
• measured equivalence relations
• invariant random subgroups
• profinite completion

“This is an attractive brisk introduction to the field, and will be a useful entry point to what is now a large and active field.” (Thomas B. Ward, zbMATH 1444.37001, 2020)

• Fakultät für Mathematik, Universität Regensburg, Regensburg, Germany

  Clara Löh

Clara Löh is Professor of Mathematics at the University of Regensburg, Germany. Her research focuses on simplicial volume and the interaction between geometric topology, geometric group theory, and measured group theory. This includes cohomological, geometric, and combinatorial methods.

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