1. M. S. Salakhitdinov and A. Hasanov, ‘‘The Dirichlet problem for generalized bi-axially symmetric Helmholtz equation,’’ Euraz. Math. J. 3 (4), 99–110 (2008).
2. M. S. Salakhitdinov and A. Hasanov, ‘‘A solution of the Neumann–Dirichlet boundary value problem for generalized bi-axially symmetric Helmholtz equation,’’ Complex Variables Ellipt. Equat. 53, 355–364 (2008).
3. E. T. Karimov, ‘‘On a boundary problem with Neumann’s condition for 3D singular elliptic equations,’’ Appl. Math. Lett. 23, 517–522 (2010).
4. E. T. Karimov and J. J. Nieto, ‘‘The Dirichlet problem for a 3D elliptic equation with two singular coefficients,’’ Comput. Math. Appl. 62, 214–224 (2011).
5. I. T. Nazipov, ‘‘Solution of the spatial Tricomi problem for a singular mixed-type equation by the method of integral equations,’’ Russ. Math. (Iz. VUZ) 3, 61–76 (2011).