@article{IVM_1964_5_a11,
     author = {S. A. Pak and E. S. \v{C}i\v{c}kin},
     title = {On the existence of upper and lower solutions of the {Cauchy} problem for a~second-order differential equation},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {91--94},
     publisher = {mathdoc},
     number = {5},
     year = {1964},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1964_5_a11/}
}