“…Let us first consider the action of Diff (M) on an algebraic equation :A, which we write in the form F(x, u) --0. As it was discussed before, A identifies a solution manifold SF in M; notice that this corresponds to a level surface of F. If we are given a vector field X E diff (M), this will generate a one-parameter subgroup To of Diff (M); in general To : (x, u) --> (g.0 (x, u), h,9 (x, u)), so that T,9 : SF --T, 9 (SF) = f (x, u) c M : F(g-,g (x, u), h-,o (x, u)) = 01 (39) The condition needed in order to have SF invariant under T,9, i.e. T,9 : SF --+ SF, is, in terms of the vector field, X : SF --> TSF; we define the symmetry group of F as GF = Ig E Diff(M): 9: SF ' SFJ (40) and-its algebra is…”