The Banks-Zaks Expansion and "Freezing" in Perturbative QCD

The recent calculation of the four-loop beta function in QCD provides further evidence that the Banks-Zaks expansion in 16half-n_f is sufficiently well behaved to be useful even for n_f=2 light flavours. This expansion inherently predicts "freezing" of the QCD couplant at low energies as a perturbative effect. We consider the e+e- and Bjorken-sum-rule cases as examples.