@article{SM_1972_16_4_a6,
     author = {A. V. Marchenko},
     title = {On~the index of boundary value problems for degenerate elliptic operators},
     journal = {Sbornik. Mathematics},
     pages = {615--634},
     year = {1972},
     volume = {16},
     number = {4},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1972_16_4_a6/}
}