“…Furthermore, Lie's classical theory represented a source for various generalizations. Among these generalizations there are the nonclassical symmetries first proposed by Bluman and Cole [43], and now part of the more general method of differential constraints [44,45], the potential symmetries [46], the nonlocal symmetries [47][48][49], the gen-eralized symmetries [5], which in turn generalize contact symmetries introduced by Lie himself, the equivalence transformations [3,[50][51][52][53][54][55][56], to quote a few. A further extension is represented by approximate symmetries [57][58][59][60][61] for differential equations containing small terms, often arising in concrete applied problems (see, for instance, [62] for an application of approximate Lie symmetries to Navier-Stokes equations).…”