  Three-body systems of scalar bosons are described in the framework of
relativistic constraint dynamics. With help of a change of variables followed
by a change of wave function, two redundant degrees of freedom get eliminated
and the mass-shell constraints can be reduced to a three-dimensional eigenvalue
problem.
  In general, this problem is complicated, but for three equal masses a drastic
simplification arises at the first post-Galilean order: the reduced wave
equation becomes tractable, and we can compute a first-order correction beyond
the nonrelativistic limit. The harmonic interaction is displayed as a toy
model.

High Energy Physics - Theory