We discuss the most general form of the electromagnetic vertex of an
off-shell pion. The framework of chiral perturbation theory is used to
illustrate the representation dependence of the electromagnetic three-point
Green's function. For that purpose we discuss the concept of field
transformations which, in comparison with the standard Gasser and Leutwyler
Lagrangian, generate equivalent Lagrangians containing additional terms at
order p^4 proportional to the lowest-order equation of motion. We consider
Compton scattering from a pion to show that calculations involving different
off-shell effects in the s- and u-channel pole diagrams may nevertheless lead
to the same on-shell Compton scattering amplitude. This is a result of the
equivalence theorem which states that two equivalent Lagrangians predict the
same S-matrix elements even though they may generate a different off-shell
behaviour of Green's functions. We conclude that off-shell effects are not only
model dependent but also representation dependent.Comment: 10 pages, RevTex, no figures, invited talk at the conference on
"Perspectives in Nuclear Physics at Intermediate Energies", Trieste, Italy,
8-12 May 199
