1. A. V. Kolmogorov and S. V. Fomin, Elements of Function Theory and Functional Analysis (Nauka, Moscow, 1972) [in Russian].
2. M. A. Naimark, Linear Differential Operators (Nauka, Moscow, 1969) [in Russian].
3. L. D. Menikhes and V. P. Tanana, “Finite-Dimension Approximation in the M. M. Lavrent’ev Method,” Sibirsk. Zh. Vychisl. Mat. 1(1), 59–66 (1998).
4. V. P. Tanana, “About Order-Optimality of the Projection Regularization Method for Solving Some Inverse Problems,” Sibirsk. Zh. Indust. Mat. 7(2), 117–132 (2004).