1. A. G. Kostyuchenko and I. S. Sargsyan, Eigenvalue Distribution (Nauka, Moscow, 1979) [in Russian].
2. R. S. Ismagilov and A. G. Kostyuchenko, Funkts. Anal. Ego Prilozh. 42(2), 11–22 (2008).
3. R. S. Ismagilov and A. G. Kostyuchenko, Funkts. Anal. Ego Prilozh. 41(1), 31–41 (2007).
4. Ya. T. Sultanaev, Differ. Uravn. 10, 1673–1683 (1974).
5. M. A. Naimark, Linear Differential Operators (Ungar, New York, 1967; Nauka, Moscow, 1969).