Let be a compact Hausdorff second countable topological group. ``second countable ``the topology of has a countable base" Irr() denotes the set of equivalence classes of irreducible representations of . The vector space for each irreducible representation of is a finite dimensional vector space over the complex numbers. Irr() is a countable set --- and we give it the discrete topology.
Let be a locally compact Hausdorff topological space with a given (continuous) action of on .
For each in , denotes the isotropy group of . If and are in the same orbit for the given action of on , then Irr() and Irr() are canonically in bijection. The usual quotient, denoted , is obtained by collapsing each orbit (for the given action of on ) to a point. The extended quotient, denoted , is constructed by replacing the orbit of (for the given action of on ) by Irr(). This talk will describe how extended quotient enters into the representation theory of Lie groups and reductive p-adic groups. For Lie groups this is due to Mackey-Higson-Afgoustidis. For reductive p-adic groups, this is due to Aubert-Baum-Plymen-Soll
Extended quotient in representation theory or Non-commutative geometry impacts representation theory
The Lefschetz fixed point theorem gives an invariant that is zero when an endomorphism is homotopic to a map with no fixed points. The converse is not true unless we refine the Lefschetz number to a richer invariant called the Reidemeister trace.
When you come to these invariants as a homotopy theorist (of a particular variety) you notice that the Reidemeister trace is a topological Hochschild homology invariant and is additive. This suggests that you should look for connections to algebraic K-theory since it is the universal home for additive invariants. In this talk I'll (start to) explain the connection between traces defined from fixed point invariants and traces defined for computing K-theory
Traces in K-theory and fixed point theory Sponsored by the Meyer Fund
Extended quotient in representation theory or Non-commutative geometry impacts representation theory