Perturbation theory in statistical mechanics and the theory of superconductivity

S. Nakajima

Victor Nikolaevich Popov

C. T. Chen-Tsai

An aiternative proof of the asymptotic exactness of the BCS theory of superconductivity within the framework of perturbation theory is made by employing the B!och Statistical mechanical pertubation theory in the Bogoliubov-Zubarev-Tserkovnikov Method.

J. R. Schrieffer

Francisco Marcelo Fernández

We discuss the application of perturbation theory to statistical mechanics both in the operator form of Schwinger and in the scalar form as a sum over states. We briefly consider and compare the use of variational parameters in both approaches. By means of a simple model we show that Schwinger perturbation theory might fail when the eigenfunctions satisfy Dirichlet boundary conditions. On the other hand, the sum over states gives accurate results.

Jean G. Valatin