Echoing and scaling in Einstein-Yang-Mills critical collapse

We confirm recent numerical results of echoing and mass scaling in the gravitational collapse of a spherical Yang-Mills field by constructing the critical solution and its perturbations as an eigenvalue problem. Because the field equations are not scale invariant, the Yang-Mills critical solution is asymptotically, rather than exactly, self-similar, but the methods for dealing with discrete self-similarity developed for the real scalar field can be generalized. We find an echoing period ?=0.73784±0.00002 and a critical exponent for the black hole mass ?=0.1964±0.0007.