Methods for deriving gauge-invariant and Lorentz invariant amplitudes of photonuclear reactions

Effective methods for taking into account the requirements of gauge and rel ativistic invariance are developed to describe electromagnetic processes on nuclei or other bound systems. The present analysis relies on the Weyl cri terion, a generalized Ward-Takahashi identity, a generalization of the Sieg ert theorem, and its local version. Connections between the field-theoretic al description of hadronic systems and the conventional approach in nuclear physics are discussed.