Title: Geometry and topology in gravitational lensing theorySpeaker: Marcus Werner (Kavli IPMU, University of Tokyo, Japan; also Mathematics Department, Duke University, US)

　　Time & Place: Wednesday, 10:00am, December 18th, Meeting Room 1714Abstract:

　　The standard theory for gravitational light deflection used in astronomy has a rich mathematical structure. I will review classic results from Morse theory on multiple images as well as a geometric interpretation of certain magnification invariants in terms of Lefschetz theory. Topological properties of the optical (or Fermat-) geometry will also be discussed, including the Gauss-Bonnet method for deflection angle computations and its recent extension to the Kerr-Randers optical geometry.