We describe results of a numerical calculation of circularly symmetric scalar
field collapse in three spacetime dimensions with 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 rAH
grows with pulse amplitude according to the formula rAH∼(A−A∗)0.81.Comment: 10 pages, 1 figure; references added, to appear in CQG(L