1. V. S. Vladimirov, Equations of Mathematical Physics [in Russian], Nauka, Moscow (1988).
2. R. Courant and D. Hilbert, Methoden der Mathematischen Physik [Russian translation], Gostekhteorizdat, Moscow (1951).
3. F. V. Atkinson, “Multiparameter spectral theory,” Bull. Amer. Math. Soc., 74, No. 1, 1–27 (1968).
4. F. V. Atkinson, Multiparameter Eigenvalue Problem, Academic Press, New York (1972).
5. B. D. Sleeman, Multiparameter Spectral Theory in Hilbert Space, Pitman, London (1988).