Algebraic Graph Theory Seminar - prof. Goulnara Arzhantseva (16.5.2025)

Friday 16.5.2025 at 13:15, Lecture room M IX

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prof. Goulnara Arzhantseva (University of Vienna):
Origami expanders

Abstract:
We construct the first measure-preserving affine actions with spectral gap on surfaces of arbitrary genus g > 1. We achieve this by finding geometric representatives of multi-twists on origami surfaces. As a major application, we construct new expanders that are coarsely distinct from the classical expanders obtained via the Laplacian as Cayley graphs of finite quotients of a group. Our methods also show that the Margulis expander, and hence the Gabber-Galil expander, is coarsely distinct from the Selberg expander. A joint work with Dawid Kielak (Oxford), Tim de Laat (Kiel), Damian Sawicki (Leuven).

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