Rz. Zhdanov, SEPARATION OF VARIABLES IN THE NONLINEAR-WAVE EQUATION, Journal of physics. A, mathematical and general, 27(9), 1994, pp. 120000291-120000297
We develop a technique making it possible to handle the problem of sep
aration of variables in nonlinear differential equations. Using it we
obtain a number of new two-dimensional nonlinear wave equations admitt
ing separation of variables and construct their exact solutions.