@article{DAN_1986_289_4_a0,
     author = {V. D. Budaev},
     title = {The property of being an unconditional basis of systems of eigen- and associated functions of a second-order operator with discontinuous coefficients},
     journal = {Doklady Akademii Nauk},
     pages = {777--780},
     year = {1986},
     volume = {289},
     number = {4},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1986_289_4_a0/}
}