@article{702174,
     title = {Problems which lead to a generalization of the concept of an ordinary nonlinear differential equation},
     booktitle = {Differential Equations and Their Applications},
     series = {GDML\_Books},
     publisher = {Publishing House of the Czechoslovak Academy of Sciences},
     address = {Prague},
     year = {1963},
     pages = {65-76},
     mrnumber = {MR0177173},
     zbl = {0151.12501},
     url = {http://dml.mathdoc.fr/item/702174}
}

Kurzweil, J. Problems which lead to a generalization of the concept of an ordinary nonlinear differential equation, dans Differential Equations and Their Applications, GDML_Books,  (1963), pp. 65-76. http://gdmltest.u-ga.fr/item/702174/