Relativistic equations for the two fermion bound states and quarkorium problem

The author discusses how relativistic considerations are necessary at least for two reasons: in order to include the light quark systems, and for a deeper understanding of why the non-relativistic approach works so well even in heavy quark systems. Three-dimensional equations which take into account the relativistic kinematics are in some sense the joining links between the non-relativistic approach and the fully covariant Bethe-Salpeter equation. There exist numerous (practically infinite) three-dimensional equations, of which the quasipotential approach is most closely related to the BS one. In this paper, the author discusses some of the key problems of quasipotential equations for two spinor constituents in the case of infinitely rising central potentials