## Geometry and Representation Theory Seminar

### Monday, November 26th, 2018

**Time:** 4:30-5:30 p.m. **Place:** Jeffery Hall 319

**Speaker:** Mike Roth (Queen's University)

**Title:** Generating Rays for the Eigencone (after Belkale and Piers)

**Abstract: ** Let G be a semisimple algebraic group. A fundamental question in the representation theory of G is knowing how to decompose the tensor product of two irreducible representations into its irreducible components, or slightly weaker, which irreducible components appear in a tensor product of two irreducible representations. The irreducible representations of G are parameterized by highest weights, vectors in ℕ^{r}, where r is the rank of G. For a highest weight λ the corresponding irreducible representation is denoted V_{λ} If one takes triples (λ, μ, ν) of highest weights such that V_{ν} appears in V_{λ} ⊗ V_{μ} then these triples generate a polyhedral cone in ℚ^{3r}, known as the eigencone (or sometimes the tensor cone). Trying to find explicit equations for the hyperplanes cutting out the eigencone is a problem with a long history, including fundamental contributions by Weyl, Gelfand, Lidskii, and Wielandt. Finally, twenty years ago, Klyachko found a set of hyperplane inequalities cutting out the eigencone in type A. Progress in the last 20 years has included finding hyperplane inequalities for the eigencones in all types, finding minimal hyperplane inequalities in all types, and finally, also finding descriptions of the linear conditions cutting out higher codimensional faces of the eigencone. Dually to their description by hyperplane inequalities, polyhedral cones may also be described by their generating rays. It is of course natural to then ask for the generating rays of the eigencone. This talk will discuss a recent paper of Belkale and Piers giving a recursive method, valid in all types, of finding generating rays for the eigencone.