@article{RM_1969_24_2_a1,
     author = {A. Yu. Levin},
     title = {Non-oscillation of solutions of the equation
},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {43--99},
     publisher = {mathdoc},
     volume = {24},
     number = {2},
     year = {1969},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/RM_1969_24_2_a1/}
}