The Klein–Gordon Equation and Differential Substitutions of the Form $v=\varphi(u,u_x,u_y)$

We present the complete classification of equations of the form $u_{xy} = f(u, u_x, u_y)$ and the Klein–Gordon equations $v_{xy} = F(v)$ connected with one another by differential substitutions $v = \varphi(u, u_x, u_y)$ such that $\varphi_{u_x}\varphi_{u_y}\neq 0$ over the ring of complex-valued variables.