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Pion contributions to the electromagnetic interaction with a nucleus.

A theoretical approach to describing the electromagnetic interaction in hadronic field theories containing charged mesons at finite baryon density is presented. Path-integral methods and bilocal auxiliary fields are used to identify covariant, gauge-invariant levels of approximation, which sum infinite classes of Feynman diagrams. At the lowest level an isolated nucleon acquires a nontrivial electromagnetic structure, and the finite-density nucleus is described as a noninteracting system of quasiparticles with that structure.