[1] A.  Bitsadze and A. Samarski: 
On some simplest generalizations of linear elliptic problems. Dokl. Akad. Nauk SSSR 185 (1969), 739–740. (Russian) 
MR 0247271[2] A.  Bitsadze: Boundary Value Problems for Elliptic Equations of Second Order. Nauka, Moscow, 1986. (Russian)
[3] D.  Gordeziani: 
On solvability of some boundary value problems for one variant of equations of thin shells. Dokl. Akad. Nauk SSSR 215 (1974), 1289–1292. (Russian) 
MR 0391657[4] D.  Gordeziani and T. Z. Djioev: The generalization of Bitsadze-Samarski problem with reference to the problems of baroclinic sea dynamics. Outlines on Physics and Chemistry of Waters of the Black Sea, IO ANSSSR, Moscow, 1978. (Russian)
[5] U.  Dini: Sulla integrazione della equazione $\Delta ^{2}u=0$. Brioschi Ann. VI (1873), 305–345. (Italian)
[6] E.  Obolashvili: 
Solution of nonlocal problems in plane elasticity theory. Current Problems of Mathematical Physics, Vol. 2. Transactions of All-Union Symposium, Gos. Univ., Tbilisi, 1987, pp. 295–302, 394. (Russian) 
MR 0991842[8] E. Obolashvili: 
P.D.E. in Clifford Analysis. Longman, Addison Wesley, 1998. 
MR 1697655[9] O. Sjöstrand: Sur une equation aux derivées partielles due type composite. Ark. Mat. Astron. Fys. 25A (1937), no. 21, 1–11.