“…pM 2n`1 , Cq with a decomposition C " E ' F into complementary Legendrian subspaces. When F is integrable, we can introduce coordinates px i , u, u i q, 1 ď i ď n, so that C " annpdu´u i dx i q and F " spantB u k u (see [9]). Now E specifies a section of M p1q Ñ M , so such structures can be regarded as complete systems of 2nd order PDE u ij " f ij px k , u, u k q up to point transformations, i.e.…”