Remarks on the Nyquist and Callen–Welton theorems

The connection of the Callen–Welton and Nyquist fluctuation–dissipation relations is considered for plasma-like classical and quantum systems. The conditions for appearance of the dissipative parameters in the equilibrium current–current correlation function are investigated. The paper presents the arguments for the restrictions of the Nyquist theorem and against violation of the Callen–Welton theorem in the quantum case.