Nonlinear coupling in low-energy meson theory

It is shown that a very large class of meson-nucleon interactions leads to the same first-order scattering equations as would be obtained by adding simple nonlinear terms to the familiar linear pseudovector coupling in the Chew-Low-Wick formalism. The class of interactions under consideration is large enough to include what one would expect to be a reasonable form for the nonrelativistic limit of the charge-symmetric pseudoscalar interaction. In the gauge-invariant extension of the theory to photoproduction, the P-wave coupling constant is not exhibited explicitly in the energy-independent S-wave part of the inhomogeneous terms of the integral equation for the transition amplitude. It is shown directly, however, that the contribution of the higher order terms in the limit of zero total energy is exactly as required for the satisfaction of the Kroll-Ruderman theorem.