Analytical approach to critical collapse in 2+1 dimensions

We present a family of time-dependent solutions to 2+1 gravity with negative cosmological constant and a massless scalar field as source. These solutions are continuously self-similar near the central singularity. We analyze linear perturbations of these solutions, and discuss the subtle question of boundary conditions. We find two growing modes, one of which corresponds to the linearization of static singular solutions, while the other describes black hole formation.