On the solvability of a multidimensional version of the Goursat problem for a second order hyperbolic equation with characteristic degeneration. Kharibegashvili, Sergo — 1997 Memoirs on Differential Equations and Mathematical Physics
Solvability of characteristic boundary-value problems for nonlinear equations with iterated wave operator in the principal part. Kharibegashvili, Sergo; Midodashvili, Bidzina — 2008 Electronic Journal of Differential Equations (EJDE) [electronic only]