1. A. V. Bitsadze and A. A. Samarskii, Dokl. Akad. Nauk SSSR 185 (4), 739–740 (1969).
2. A. V. Bitsadze, Some Classes of Partial Differential Equations (Nauka, Moscow, 1981; Gordon and Breach, New York, 1988).
3. T. Sh. Kal’menov and D. Suragan, Dokl. Math. 78 (3), 913–915 (2008).
4. T. Sh. Kal’menov and D. K. Suragan, Dokl. Math. 80 (2), 646–649 (2009).
5. T. Sh. Kal’menov and D. Suragan, Operator Theory Adv. Appl. 216, 187–210 (2011).