1. Samarskii, A.A.,Differents. Uravn., 1980, vol. 16, no. 11, pp. 1221–1228.
2. Cannon, J.R.,Quart. Appl. Math., 1963, vol. 21, no. 2, pp. 155–160.
3. Ionkin, N.I.,Differents. Uravn., 1977, vol. 13, no. 2, pp. 294–304.
4. Bouziani, A. and Benouar, N.-E.,C.R. Acad. Sci. Paris, 1995, vol. 321, no. 1, pp. 1177–1182.
5. Nakhushev, A.M.,Differents. Uravn., 1982, vol. 18, no. 1, pp. 72–81.