Holographic positive energy theorems in three-dimensional gravity
  The covariant phase space of three-dimensional asymptotically flat and anti-de Sitter gravity is controlled by well-understood coadjoint orbits of the Virasoro group. Detailed knowledge on the behavior of the energy functional on these orbits can be used to discuss positive energy theorems. 
Glenn Barnich and Blagoje Oblak
hep-th gr-qc