@article{DAN_1990_310_4_a1,
     author = {M. G. Gasymov and B. M. Levitan},
     title = {On expansion in products of special solutions of two {Sturm--Liouville} equations},
     journal = {Doklady Akademii Nauk},
     pages = {781--784},
     publisher = {mathdoc},
     volume = {310},
     number = {4},
     year = {1990},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1990_310_4_a1/}
}