STRING EXPANSION AS LARGE N EXPANSION OF GAUGE-THEORIES

We consider string perturbative expansion in the presence of D-branes imbedded in orbifolded space-time. In the regime where the string coup ling is weak and alpha' --> 0, the string perturbative expansion coinc ides with 't Hooft's large N expansion. We specifically concentrate on theories with d = 4 and N = 0, 1, 2, 4, and use world-sheet orbifold techniques to prove vanishing theorems for the field theory beta-funct ions to all orders in perturbation theory in the large N limit. This i s in accord with recent predictions. (C) 1998 Elsevier Science B.V.