@article{RM_1975_30_1_a19,
     author = {A. Yu. Levin and G. D. Stepanov},
     title = {One-dimensional boundary value problems with operators that do not lower the number of sign alternations},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     publisher = {mathdoc},
     volume = {30},
     number = {1},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1975_30_1_a19/}
}