1. M. L. Gorbachuk, “On analytic solutions of differential-operator equations,” Ukr. Mat. Zh., 52, No.5, 596–607 (2000).
2. N. P. Kuptsov, “Direct and inverse theorems of approximation theory and semigroups of operators,” Usp. Mat. Nauk., 23, Issue 4, 118–178 (1968).
3. N. I. Akhiezer and I. M. Glazman, Theory of Linear Operators in a Hilbert Space [in Russian], Nauka, Moscow (1966).
4. N. I. Akhiezer, Lectures on Relativity Theory [in Russian], Nauka, Moscow (1965).
5. M. L. Gorbachuk and V. I. Gorbachuk, “Spaces of infinitely differentiable vectors of a closed operator and their application to problems of approximation,” Usp. Mat. Nauk, 48, Issue 4, 180 (1993).