1. J. L. Lions, Equations differentielles operationelles [Operational Differential Equations], Springer-Verlag (1961).
2. F. S. Rofe-Beketov, Eigenfunction expansion of infinite systems of differential equations in the nonself-adjoint and self-adjoint cases, Matem. Sb.,51 (93), No. 3, 293?342 (1960).
3. R. Z. Khalilova, Asymptotics of a spectral function, Izv. AN Azerb. SSR. ser. fiziko-tekhn. i matem., Nauk, No. 5, 41?49 (1965).
4. E. C. Titchmarsh, Eigenfunction Expansions [Russian translation], Vol. I, IL, Moscow (1960). [English edition: Clarendon Press, Oxford, 1946?58].
5. E. C. Titchmarch, Eigenfunction Expansions [Russian translation], Vol. II, IL, Moscow (1961).