Two-particle equations for fermions in models with trilinear interaction

Two-particle equations for a spinor field (the Edwards and Bethe–Salpeter equations) are obtained as a consequence of the Sehwinger equations by the method of the second Legendre transformation. For a theory with trilinear interaction which is local in the fermions, a relation between the kernel of the two-particle equations and the propagator is obtained and its consequences considered.