[1] Chrastina J.: Another approach to the classical calculus of variations, I. Lagrange problem. Arch. Math. 4 (1982), p. 205-218. MR 0685806

[2] Krupka D.: Lepagean forms in higher order variational theory. Pгoc. IUTAM-ISIMM Sympos. on Modern Developments in Analyt. Mech., Turin (1982), to appear. MR 0773488