1. Ionkin, N.I., Differ. Uravn., 1977, vol. 13, no. 2, pp. 294–304.
2. Ionkin, N.I. and Moiseev, E.I., Differ. Uravn., 1979, vol. 15, no. 7, pp. 1284–1295.
3. Bari, N.K., Uch. Zap. Moskov. Gos. Univ., 1951, vol. 148,no. 4, pp. 69–107.
4. Il’in, V.A., Dokl. Akad. Nauk SSSR, 1976, vol. 227, no. 4, pp. 796–799.
5. Makin, A.S., Differ. Uravn., 1999, vol. 35, no. 8, pp. 1058–1066.