@article{SM_1971_14_4_a1,
     author = {A. L. Chistyakov},
     title = {Defect indices of $J_m$-matrices and of differential operators with polynomial coefficients},
     journal = {Sbornik. Mathematics},
     pages = {471--500},
     publisher = {mathdoc},
     volume = {14},
     number = {4},
     year = {1971},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1971_14_4_a1/}
}