Path integrals in configuration space in weakly relativistic many-body theory

The functional method of quantizing weakly relativistic theories is considered. It is shown that in the general case of systems with Lagrangian nonquadratic in the velocities the Green's function can be represented in the form of the regular part of a path integral in the configuration space. On this basis, a functional formulation of equilibrium statistical mechanics that does not require a Hamiltonian description of the system is developed. The results are used to determine the free energy of a system of charged particles described by the Darwin Lagrangian.