Relative to the interpretation of terms of the Schrödinger perturbation theory series

An approach different from those ordinarily expounded is proposed for the construction of the Schrödinger perturbation theory. This approach sets up a functional dependence between the energy corrections and the coefficients characterizing the corrections to the wave functions. It is shown that within the framework of the Schrödinger theory these coefficients are determined by extremum conditions of fourth-order corrections to the energy. In the Schrödinger approximation the fourth-order energy corrections reach their extremal values. The approach developed includes a proof of the Dalgarno and Stewart theorem that says that the energy can be estimated to the order (2n + 1) if the wave function is known to n-th-order accuracy.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 18–20, February, 1982.