Khabirov  S. V.
          A property of the defining equations for the Lie  algebra in the group classification problem for wave equations
        We solve the group classification problem for nonlinear hyperbolic systems  of differential equations. The admissible continuous group of transformations  has the Lie algebra of dimension less than 5. This main statement follows from  the principal property of the defining equations of the admissible Lie algebra:  the commutator of two solutions is a solution. Using equivalence  transformations we classify nonlinear systems in accordance with the well-known  Lie algebra structures of dimension 3 and 4.