GOA seminar: Peterson

Speaker: Jesse Peterson (University of Waterloo)

Title: A hierarchy of Haagerup-type approximation properties

Abstract:  The Haagerup property for groups and von Neumann algebras is a well-studied approximation property, allowing for certain deformability phenomena to extend beyond the amenable realm and into the realm of free group factors. We introduce successive weakenings of the Haagerup property, indexed by the ordinal numbers. We show that for each countable ordinal $\alpha$, the $\alpha$-Haagerup property, like the Haagerup property itself, is an invariant of the group von Neumann algebra and passes to von Neumann subalgebras. We use this notion to give a new proof of Ozawa's theorem that there is no universal separable $\mathrm{II}_1$ factor. This is joint work with Fabian Salinas.