Bethe Ansatz in Quantum Mechanics. 1. The Inverse Method of Separation of Variables

In this paper we formulate a general method for building completely integrable quantum systems. The method is based on the use of the so-called multi-parameter spectral equations, i.e. equations with several spectral parameters. We show that any such equation, after eliminating some spectral parameters by means of the so-called inverse procedure of separation of variables can be reduced to a certain completely integrable model. Starting with exactly or quasi-exactly solvable multi-parameter spectral equations we, respectively, obtain exactly or quasi-exactly solvable integrable models.