Chiral symmetry breaking and perturbative QCD

Exact results are given for the propagators of axial and vector divergences at the two-loop level of dimensionally regularized quantum chromodynamics. We prove that any combination of propagators that vanishes at zero momentum by virtue of Ward identities is free of subtractions and satisfies a superconvergence relation. Sum rules for other combinations will, however, involve unknown non-perturbative constants. (orig.)