Scalar field collapse in three-dimensional AdS spacetime

We describe the results of a numerical calculation of circularly symmetric scalar field collapse in three spacetime dimensions with a negative cosmological constant. The procedure uses a double null formulation of the Einstein-scalar equations. We see evidence of black hole formation on first implosion of a scalar pulse if the initial pulse amplitude A is greater than a critical value A_*. Sufficiently near criticality the apparent horizon radius r_AH grows with pulse amplitude according to the formula r_AH~(A-A_*)^0.81.