Quasirelativistic systems of interacting particles

On the basis of the general principles of the relativistic theory of direct interactions (causality, Poincare invariance, and separability), the elements of a quasirelativistic (classical and quantum) mechanics of a system of particles are presented. This makes it possible to take into account small relativistic effects in objects possessing internal structure. The relationship between the different formalisms of the theory is analyzed, the physical meaning of the variables is discussed, the general form of the post-Newtonian interaction Lagrangians and Hamiltonians is found, and their physical interpretation and relation to field approaches are considered. The introduction of quasirelativistic center-of-mass variables solves the problem of separating the motion of a weakly relativistic system of particles as a whole from its internal motion in the case of an arbitrary interaction