Exact formula for nondiagonal Green's functions in condensed-matter physics

An exact formula for the nondiagonal part of the one-particle temperature Green’s function is obtained within the framework of the nonperturbative canonical formulation of the finite-temperature generalized Ward-Takahashi relation. It is shown that the Landau-level Green’s function for two-dimensional electrons has a nonvanishing nondiagonal part if the electron-electron interaction is included.