is associated to one of the one-parameter symmetry groups. It acts on 4 variables, namely a , b , c and P . One can easily verify that δ δ f 1 = f 1 − − f 2 and δ δ f 2 = 0 . Thus the relations δ δ f 1 = δ δ f 2 = 0 are satisfied for any Z in R 6 that vanishes the algebraic system.