Group classification of nonlinear wave equations
  We perform complete group classification of the general class of quasi linear wave equations in two variables. This class may be seen as a broad generalization of the nonlinear d'Alembert, Liouville, sin/sinh-Gordon and Tzitzeica equations. In this way we derived a number of new genuinely nonlinear invariant models with high symmetry properties. In particular, we obtain four classes of nonlinear wave equations admitting five-dimensional invariance groups. Applying the symmetry reduction technique we construct multi-parameter families of exact solutions of those wave equations. 
V.I.Lagno (State Pedagogical University, Poltava), R.Z. Zhdanov   (Institute of Mathematics, Kiev) and O.Magda (Institute of...