A Precise Definition of Reduction of Partial Differential Equations

We give a comprehensive analysis of interrelations between the basic concepts of the modern theory of symmetry classical and non-classical.reductions of partial differential equations. Using the introduced definition of reduction of differential equations we establish equivalence of the non-classical conditional symmetry.and direct Ansatz.approaches to reduction of partial differential equations. As an illustration we give an example of non-classical reduction of the nonlinear wave equation in 1q3 dimensions. The conditional symmetry approach when applied to the equation in question yields a number of non-Lie reductions which are far-reaching generalizations of the well-known symmetry reductions of the nonlinear wave equations.