Nonperturbative study of generalized ladder graphs in a \phi^2\chi   theory
  The Feynman-Schwinger representation is used to construct scalar-scalar bound states for the set of all ladder and crossed-ladder graphs in a \phi^2\chi theory in (3+1) dimensions. The results are compared to those of the usual Bethe-Salpeter equation in the ladder approximation and of several quasi-potential equations. Particularly for large couplings, the ladder predictions are seen to underestimate the binding energy significantly as compared to the generalized ladder case, whereas the solutions of the quasi-potential equations provide a better correspondence. Results for the calculated bound state wave functions are also presented. 
Taco Nieuwenhuis and J.A. Tjon (Institute for Theoretical Physics,   University of Utrecht, the Netherlands)