Gauge Formulation of the Spinning Black Hole in (2+1)-Dimensional   Anti-de Sitter Space
  We compute the group element of SO(2,2) associated with the spinning black hole found by Ba\~nados, Teitelboim and Zanelli in (2+1)-dimensional anti-de Sitter space-time. We show that their metric is built with SO(2,2) gauge invariant quantities and satisfies Einstein's equations with negative cosmological constant everywhere except at $r=0$. Moreover, although the metric is singular on the horizons, the group element is continuous and possesses a kink there. 
Daniel Cangemi, Martin Leblanc and Robert B. Mann