“…As well known, any vector field X = ξ i ∂/∂x i + ζ a ∂/∂u a is a conditional symmetry for a differential equation (or a system thereof) ∆ = 0 if the system ∆ = 0 enlarged with the invariance condition (13) admits some solution. If this is the case, a conditional symmetry allows the reduction of the initial equation into a reduced form, and in this way one can obtain other invariant solutions: see [27,28,29,30,31,32]. See also [35,36,37,38,39,40,41,42,43] for careful discussions about various related problems and reduction procedures.…”