GAUGE-INVARIANCE, THE QUANTUM ACTION PRINCIPLE, AND THE RENORMALIZATION-GROUP

If the Wilsonian renormalization group (RG) is formulated with a cutof f that breaks gauge invariance, then gauge invariance may be recovered only once the cutoff is removed and only once a set of effective Ward identities is imposed. We show that an effective quantum action princ iple can be formulated in perturbation theory which enables the effect ive Ward identities to be solved order by order, even if the theory re quires nonvanishing subtraction points, The difficulties encountered w ith nonperturbative approximations are briefly discussed.