1. G. A. Bliss,Lectures on the calculus of variations, University of Chicago Press, Chicago (1946).
2. H. V. Craig,On the solution of the Euler equations for their highest derivatives, Bull. Amer. Math. Soc.36 (1930), pp. 558–562.
3. H.S.P. Grässer,Hamilton-Jacobi theory for parameter-invariant problems of the second order in the calculus of variations, Ph. D. thesis, University of South Africa, Pretoria (1966).
4. E. Kamke,Differentialgleichungen 1,Gewöhnliche Differentialgleichungen, 5. Auflage, Akademische Verlagsgesellschaft Geest & Portig, Leipzig (1964).
5. L. H. McFarlan,Problems of the calculus of variations in several dependent variables and their derivatives of various orders, Tôhoku Math. J.33 (1931), pp. 204–218.