Some special types of solutions of a class of the (N+1)(N+1)-dimensional nonlinear wave equations

Some special types of solutions with arbitrary functions for the (N+1)(N+1)-dimensional sine–cosine-Gordon equation, the (N+1)(N+1)-dimensional double sinh-Gordon equation, and the (N+1)(N+1)-dimensional sinh–cosinh-Gordon equations are constructed by means of the separation transformation approach. As an illustrative sample, the properties of some solutions are shown by their figures.