Contents: Mixed problems for the wave equation in co-ordinate corner with additional condition on edge; Well-posedness of mixed problems for wave equation and general hyperbolic equation of second order in co-ordinate corner; Ill-posedness examples in mixed problem; Mixed problem for wave equation in co-ordinate corner -- problem (B0). Solvability condition. Exact solution. A priori estimate in W1/2 (R ); Obtaining of a priori estimate in mixed problems for the multidimensional wave equation.
