1. H. von Helmholtz, Ueber die physikalische Bedeutung des Princips der kleinsten Wirkung. J. Reine Angewandte Math. 100, 18 (1887)
2. J. Douglas, Solution of the inverse problem of the calculus of variations. Trans. Am. Math. Soc. 50, 71–128 (1941)
3. W. Sarlet, The Helmholtz conditions revisited. A new approach to the inverse problem of Lagrangian dynamics. J. Phys. A: Math. Gen. 15, 1503–1517 (1982)
4. G.E. Prince, D. King, “The inverse problem in the calculus of variations: nonexistence of Lagrangians”, in Differential Geometric Methods on Mechanics and Field Theory: Volume in Honour of Willy Sarlet, F. Cantrijn and B. Langerock editors (Academia Press, Gent, Belgium, 2007), pp. 131–140
5. D.V. Zenkov (ed.), The inverse problem of the calculus of variations: local and global theory (Atlantis Press, Paris, 2015)