A 2-INSTANTON TEST OF THE EXACT SOLUTION OF N=2 SUPERSYMMETRIC QCD

The exact solution of N = 2 supersymmetric QCD found by Seiberg and Wi tten includes a prediction for all multiinstanton contributions to the low-energy dynamics. In the case where the matter hypermultiplets are massless, the leading non-perturbative contribution at weak coupling comes from the two-instanton sector. We calculate this contribution fr om first principles using the exact two-instanton solution of the self -dual Yang-Mills equation found by Atiyah, Drinfeld, Hitchin and Manin . We find exact agreement with the predictions of Seiberg and Witten f or all numbers of flavours, We also confirm their predictions for the case of massive hypermultiplets.