University of Sydney Algebra Seminar

Anne Thomas (University of Sydney)

Friday 7 August, 12-1pm, Place: Carslaw 375

Affine Deligne-Lusztig varieties and the geometry of Euclidean reflection groups

Let $G$ be a reductive group such as $S{L}_n$ over the field $F=k((t))$, where $k$ is an algebraic closure of a finite field, and let $W$ be the affine Weyl group of $G(F)$. The associated affine Deligne-Lusztig varieties $X_x(b)$ were introduced by Rapoport. These are indexed by elements $x\in W$ and $b\in G(F)$, and are related to many important concepts in algebraic geometry over fields of positive characteristic. Basic questions about the varieties $X_x(b)$ which have remained largely open include when they are nonempty, and if nonempty, their dimension. For these questions, it suffices to consider elements $x$ and $b$ both in $W$. We use techniques inspired by geometric group theory and representation theory to address these questions in the case that $b$ is a translation. Our approach is constructive and type-free, sheds new light on the reasons for existing results and conjectures, and reveals new patterns. Since we work only in the standard apartment of the affine building for $G(F)$, which is just the tessellation of Euclidean space induced by the action of the reflection group $W$, our results also hold over the p-adics. We obtain applications to class polynomials of affine Hecke algebras and to reflection length in $W$. This is joint work with Elizabeth Milicevic (Haverford) and Petra Schwer (Karlsruhe).