no. 2
Geometry of some simple nonlinear differential operators
Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 13 (1986) no. 2, pp. 299-346 
@article{ASNSP_1986_4_13_2_299_0,
     author = {Mc Kean, H. P. and Scovel, J. C.},
     title = {Geometry of some simple nonlinear differential operators},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {299--346},
     year = {1986},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 13},
     number = {2},
     mrnumber = {876127},
     zbl = {0648.47048},
     language = {en},
     url = {https://www.numdam.org/item/ASNSP_1986_4_13_2_299_0/}
}
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Mc Kean, H. P.; Scovel, J. C. Geometry of some simple nonlinear differential operators. Annali della Scuola Normale Superiore di Pisa - Classe di Scienze, Série 4, Tome 13 (1986) no. 2, pp. 299-346. https://www.numdam.org/item/ASNSP_1986_4_13_2_299_0/

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