On Gauge Invariance and Equivalence Theorems

The gauge invariance theorem and the equivalence theorems are studied in the Feynman-Dyson formalism. Our results may be of considerable use in reducing complicated calculations. When the Dyson diagram contains a closed loop, certain ambiguities appear as usual. From the standpoint that surface integrals in the momentum space should be estimated accurately, however, they are easily removed by a simple prescription. As an application of our method, the two-gamma-decay of a neutral meson is reexamined and entirely consistent results are obtained.