Correlation theory for one dynamical variable: Application to EuO and EuS

A self-consistent correlation theory is developed which is a systematic generalization of the random-phase approximation and with an equally wide range of applicability. For simplicity the theory is here given in detail for one dynamical variable. For a test of the theory static and dynamic magnetic properties for EuO and EuS for $T\ge T_c$ are calculated. Quantitative agreement with the comprehensive and detailed experimental data is obtained for $T\ge 1.02T_c$ with no adjustable parameters. A dependence on the lattice and spin dimension as well as the value of the spin is obtained and is in agreement with high-temperature-expansion results.