The flat limit of three dimensional asymptotically anti-de Sitter   spacetimes
  In order to get a better understanding of holographic properties of gravitational theories with a vanishing cosmological constant, we analyze in detail the relation between asymptotically anti-de Sitter and asymptotically flat spacetimes in three dimensions. This relation is somewhat subtle because the limit of vanishing cosmological constant cannot be naively taken in standard Fefferman-Graham coordinates. After reformulating the standard anti-de Sitter results in Robinson-Trautman coordinates, a suitably modified Penrose limit is shown to connect both asymptotic regimes. 
Glenn Barnich, Andr\'es Gomberoff, Hern\'an A. Gonz\'alez
gr-qc hep-th