Nonlinear hyperbolic differential equations related with the Klein-Gordon equation by differential substitutions
We present a complete classification of nonlinear hyperbolic differential equations in two independent variables $u_{xy}=f(u,u_x,u_y)$ reduced to the Klein-Gordon equation $v_{xy} = F(v)$ by differential substi\-tutions of the special form ${v = \varphi(u, u_x)}$