On the theory of the conductivity of crystals

On the basis of Kubo's formula, Green's superoperators are used to calculate the electrical conductivity of crystals in the framework of perturbation theory. The proposed method does not require summation of infinite series (in the limit $\omega\rightarrow 0$). The mobility of a Frölich polaron in the case of weak coupling and of a polaron in Feynman's model is considered at low temperatures.