Geodesic
Approximation of the Pfaff equations of several variables
Matematičeskoe obrazovanie, Tome 100 (2021) no. 4, pp. 57-62.

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The paper proposes a method for constructing approximate integrable analogs of non-integrable Pfaff equations. A special method for checking the exactness of the Pfaffian differential form is considered and an analytic form of the potential function is given without applying the theory of curvilinear integrals. 
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     author = {V. V. Ivlev and E. A. Krivoshey},
     title = {Approximation of the {Pfaff} equations of several variables},
     journal = {Matemati\v{c}eskoe obrazovanie},
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V. V. Ivlev; E. A. Krivoshey. Approximation of the Pfaff equations of several variables. Matematičeskoe obrazovanie, Tome 100 (2021) no. 4, pp. 57-62. http://geodesic.mathdoc.fr/item/MO_2021_100_4_a8/

[1] V. V. Stepanov, Kurs differentsialnykh uravnenii, Nauka, M., 1958

[2] V. V. Ivlev, E. M. Arkhipova, “Ob odnom priblizhenii uravnenii Pfaffa”, Matematicheskoe obrazovanie, 2018, no. 4 (88)

[3] V. V. Ivlev, Matematicheskii analiz, Izbrannoe, Izdatelstvo “Ikar”, M., 2018