Separation of variables and exact solutions of generalized nonlinear Klein-Gordon equations

In this paper, the generalized conditional symmetry approach is developed to study the separation of variables for generalized nonlinear Klein-Gordon equations. We derive a complete list of canonical forms for a generalized nonlinear Klein-Gordon equation and a system of generalized nonlinear Klein-Gordon equations that submit ti, separation of variables in some coordinates. As a result, some exact solutions to the Bullough-Dodd equation, Liouville equation, Sine-Gordon equation and Sinh-Gordon equation are obtained. A symmetry group interpretation of the known results concerning separation of variables with the scalar Klein-Gordon equation is also given.