Quasirelativistic systems of interacting particles

On the base of general principles of the relativistic direct interaction theory (causality, Poincare invariance and se-- parability) the fundamentals of qUasirelativistic (calssical and quantum) particle system mechanics are given enabling to account small relativistic effects in ob ects having an inner structure. Relations among different formalisms of the theory is analysed, physical meaning of variables is discussed, the general form of post-Newtonian interaction Lagrangians and Hamiltonians is found, their physical interpretation and connection with field approaches are considered. For weakly relativistic particle system with arbitrary interaction the problem of separating the motion of the system as a whole from its inner motion by means of introduction of quasirelativistic center-of-mass variables is solved