@article{RM_1966_21_5_a20,
     author = {I. M. Koval'chik},
     title = {A~differential-variational equation},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     publisher = {mathdoc},
     volume = {21},
     number = {5},
     year = {1966},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1966_21_5_a20/}
}