# Rigidity

Hausdorff Trimester Program

September - December 2009

**Organizers:** Wolfgang Lück and Nicolas Monod

There is a long list of rigidity phenomena whose discovery was a striking surprise and triggered a host of fruitful and lasting activities in mathematics. In many instances, these very discoveries led to outstanding open problems currently subject of intense activity.

The goal of this Trimester Program was to study and link rigidity phenomena in different areas of pure mathematics. The main focus was on:

- Mostow-Margulis-Zimmer Rigidity
- Rigidity in Topology
- von Neumann Rigidity

The Trimester Program brought together experts from, used methods from and contributed substantially to the following fields:

- Algebraic K-and L-theory
- Surgery theory
- L
^{2}-methods - Finite von Neumann algebras and measure theory
- Geometric group theory
- Cohomological algebra
- Bounded cohomology
- Asymptotic methods on semi-simple groups
- Multiplicative ergodic theory
- Functional-analytic aspects of group representations