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2. B. M. Levitan and V. V. Zhikov, Almost Periodic Functions and Differential Equations (Nauka, Moscow, 1989) [in Russian].
3. V. P. Mikhailov, Partial Differential Equations (Nauka, Moscow, 1976) [in Russian].
4. S. L. Sobolev, Some Applications of Functional Analysis in Mathematical Physics (Nauka, Moscow, 1988) [in Russian].
5. V. S. Vladimirov, Equations of Mathematical Physics (Nauka, Moscow, 1976) [in Russian].