CROSSING-SYMMETRICAL 2-PARTICLE REDUCTION OF 4-POINT VERTEX

We develop a nonperturbative reduction scheme for the four-point verte x T of a generic field theory with interactions among bosons, fermions , and antifermions. We exhibit integral equations which express, in a manifestly crossing-symmetric way, the full vertex in terms of its two -particle irreducible part, together with the dressed three-point vert ices and two-point propagators. This scheme generalizes the usual summ ation of ladder or bubble diagrams, thus providing for the consistent summation of a larger subset of all diagrams.