1. Shilov, G.E.: Generalized Functions and Partial Differential Equations. Gordon and Breach, New York (1968)
2. Naimark, M.A.: Linear Differential Operators. Nauka, Moscow, Russia (1969), in Russian
3. Stakgold, I.: Green’s Functions and Boundary Value Problems. Wiley, New York (1979)
4. Tikhonov, A.N., Vasil’eva, A.B., Sveshnikov, A.G.: Differential Equations. Nauka, Moscow, Russia (1980), in Russian
5. Krein, S.G.: Linear Equations in Banach Space. Nauka, Moscow, Russia (1971), in Russian