1. Benfield, M.: Some geometric aspects of hyperbolic conservation laws. PhD thesis, NSCU (2016).
https://repository.lib.ncsu.edu/handle/1840.16/11372
2. Benfield, M., Jenssen, H.K., Kogan, I.A.: On two integrability theorems of Darboux. Unpublished manuscript (2017).
arxiv:1709.07473
3. Bryant, R.L., Chern, S.S., Gardner, R.B., Goldschmidt, H.L., Griffiths, P.A.: Exterior Differential Systems. Mathematical Sciences Research Institute Publications, vol. 18. Springer, New York (1991). MR1083148 (92h:58007)
4. Darboux, G.: Leçons sur les systèmes orthogonaux et les coordonnées curvilignes. Gauthier-Villars, Paris (1910)
5. Hartman, P.: Ordinary Differential Equations, Classics in Applied Mathematics, vol. 38. Society for Industrial and Applied Mathematics (SIAM), Philadelphia, PA 2002. Corrected reprint of the second (1982) edition [Birkhäuser, Boston, MA; MR0658490 (83e:34002)]; With a foreword by Peter Bates. MR1929104