On the covariant relativistic separable kernel
  Two different methods of the covariant relativistic separable kernel construction in the Bethe-Salpeter approach are considered. One of them leads in the center-of-mass system of two particles to the quasipotential equation. The constructed 4-dimensional covariant functions are used to reproduce neutron-proton phase shifts for total angular momenta $J=0,1$. Obtained results are compared with other model calculations. 
S.G. Bondarenko, V.V. Burov, E.P. Rogochaya, and Y. Yanev