“…In this paper, our research is mainly concentrated on rigorous and exhaustive group classification of the whole class (1) with a detailed proof and Lie symmetry reduction of some truly nonlinear 'variable-coefficient' wave equation (equation with H u k x = 0) from this class. The method used here is a combination of the Lie-Ovsiannikov method [1,49], additional equivalent transformation and furcate split method which was present in recent paper [43,52] and had been applied to investigating a number of different group classification problem [13,28,34,35,51,58,59]. Nonclassical symmetries of the class of generalized quasi-linear wave equations (1) are also investigated.…”