  This is one of the two papers where the optimized perturbation theory was
first formulated. The other paper is published in Theor. Math. Phys. 28,
652--660 (1976). The main idea of the theory is to reorganize the perturbative
sequence by introducing control functions, defined by optimization conditions,
so that the reorganized approximation sequence be convergent. In the present
paper, the theory of perturbations is suggested for statistical systems in the
absence of small interaction parameters. A new form is advanced for
self-consistent conditions defining the optimal parameters for trial Green
functions in iterating nonlinear propagator equations. Superharmonic,
semiharmonic, and pseudoharmonic approximations for a molecular crystal are
considered as examples.
