  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 Delta = 0.73784 +/- 0.00002 and critical
exponent for the black hole mass gamma = 0.1964 +/- 0.0007.
