I am a Professor at the University of Maryland in College Park, on leave for AY 2023-24 Here is my curriculum vitae as of March 2024.

Research Interests

• differential-geometric aspects of rigidity
• Lorentzian geometry
• conformal pseudo-Riemannian geometry and parabolic Cartan geometries in general
• smooth dynamics

Recent papers:

• Classification of closed conformally flat Lorentzian manifolds with unipotent holonomy, with William Goldman and Nakyung Lee
  pdf
• Smooth and analytic actions of SL(n,R) and SL(n,Z) on closed n-dimensional manifolds, with David Fisher
  pdf also posted at http://arxiv.org/abs/2210.09516
  to appear in Kyoto Journal of Mathematics
  
• The Lorentzian Lichnerowicz Conjecture for real-analytic, three-dimensional manifolds, with Charles Frances
  pdf also posted at http://arxiv.org/abs/2108.07215
  to appear in Journal für die reine und angewandte Mathematik (Crelle)
• The conformal group of a compact, simply connected Lorentzian manifold, with Vincent Pecastaing
  pdf
  Journal of the American Mathematical Society 35 no. 1 (2022) 81-122.
• Survey article on Rigidity of transformation groups in differential geometry
  pdf
  Notices of the American Mathematical Society 68 no. 5 (2021) 721--732.
• C¹ deformations of almost-Grassmannian structures with strongly essential symmetry, with Andreas Čap
  pdf
  Transformation Groups 26 no. 4 (2021) 1169--1187.

All older papers:

Recent and upcoming talks and conferences

• Group actions and rigidity: around the Zimmer program Institut Henri Poincaré, Paris, June 2024.
• Centre International de Rencontres Mathématiques Research School on Group actions and rigidity: around the Zimmer program, April 2024.
• Max Planck Institute for Mathematics in the Sciences: Geometry Seminar, February 2024.
• Westfälische Wilhelms-Universität Münster: Oberseminar Differentialgeometrie, October 2023.
• University of Luxembourg: Geometry-Topology Seminar, October 2023.
• Groups and Rigidity around the Zimmer Program, Ventotene, Italy, September 2023.
• Rice University: Geometry Seminar, March 2023.
• Differential Geometry and Applications, Hradec Králové, Czechia, July 2022.
• Geometric Structures, Compactifications, and Group Actions (co-organizer), Université de Strasbourg, June 2022
• Global rigidity of actions by higher-rank groups, American Institute for Mathematics, May 2022
• Workshop on Cartan Connections, Geometry of Homogeneous Spaces, and Dynamics (co-organizer) Erwin Schrödinger Institute, July 2011
      Abstracts for week 2, with some links to slides
      I am a guest editor for the ensuing special issue of the Central European Journal of Mathematics

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  Teaching

  I am on leave from the University of Maryland in AY 2023-24. In 2015, I ran a module in the MAPS-REU on "Local isometries of 3-dimensional spacetimes." See below for a picture of our group. In 2013, I read about the social psychology of evaluations of teaching effectiveness, and produced this short essay.

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  Some Pictures and Links

  • Portrait of incoming graduate students, University of Chicago, September 1999
  • A gathering of Chicago mathematicians, June 2010
  • MAPS-REU Team Geometry, 2015