Geodesic
Systems of Pfaffian equations with nonanalytic coefficients
Trudy Matematicheskogo Instituta imeni V.A. Steklova, Tome 30 (1975) no. 1 
Yu. V. Titarenko. Systems of Pfaffian equations with nonanalytic coefficients. Trudy Matematicheskogo Instituta imeni V.A. Steklova, Tome 30 (1975) no. 1. http://geodesic.mathdoc.fr/item/RM_1975_30_1_a29/
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     title = {Systems of {Pfaffian} equations with nonanalytic coefficients},
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[1] J. A. Schouten, W. v. d. Kulk, “Beiträge zur Theorie der Systeme Pfaffscher Gleichungen. I”, Nederl. Akad. Wetensch., Proc., 43 (1940), 19–31; “II”, 179–188 ; “III”, 453–462 ; “IV”, 674–686 | MR | MR | Zbl | MR

[2] E. Cartan, “Sur un theoreme de J. A. Schouten et W. v. d. Kulk”, C.R. Acad. Sci. Paris, 211:4 (1940), 21–24 | MR

[3] E. Kartan, Vneshnie differentsialnye sistemy i ikh geometricheskie prilozheniya, MGU, 1962

[4] J. A. Schouen, W. v. d. Kulk, Pfaff's problem and its generalizations, Oxford, 1949

[5] K. Godbiion, Differentsialnaya geometriya i analiticheskaya mekhanika, Mir, M., 1973