[1] E. Cartan: La Geometria da los Ecuaciones Diferenciales de Tracer Orden. Rev. Mat. Hispano-Amer. t. 4 (1941), 1 - 31.

[2] H. Flanders: A Method of General Linear Frames in Riemannian Geometry I. Pacific J. Math. 3 (1953), 551-565. DOI 10.2140/pjm.1953.3.551 | MR 0056347 | Zbl 0051.39301

[3] H. Flanders: Development of an Extended Exterior Differential Calculus. Trans. Amer. Math. Soc. 75 (1953), 311-326. DOI 10.1090/S0002-9947-1953-0057005-8 | MR 0057005 | Zbl 0052.17901

[4] H. Flanders: Differential Forms with Applications to the Physical Sciences. Academic Press, New York-London, 1963. MR 0162198 | Zbl 0112.32003

[5] R. Gardner: Differential Geometric Methods in Partial Differential Equations. Thesis, University of California at Berkely, Unpublished, 1965.

[6] R. Gardner: Invariants of Pfaffian Systems. Trans. Amer. Math. Soc. 126 (1967), 514-533. DOI 10.1090/S0002-9947-1967-0211352-5 | MR 0211352 | Zbl 0161.41301