1. M. A. Naimark, Linear Differential Operators (Moscow, 1969) [in Russian].
2. A. A. Shkalikov, Vestn. Mosk. Univ., Ser. 1. Mat. Mekh., No. 6, 12–20 (1982).
3. V. A. Il’in, Differ. Uravn. 30(9), 1516–1529 (1994).
4. V. A. Il’in, in Selected Topics in Mathematics, Mechanics, and Their Applications (Moscow, 1999), pp. 223–229 [in Russian].
5. V. A. Il’in, in Program Systems (Moscow, 1999), pp. 36–43 [in Russian].