@article{DE_1976_12_9_a0,
     author = {E. S. Birger and G. A. Kalyabin},
     title = {Theory of {Weyl} circles in the case of a nonselfadjoint system of second order differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1531--1540},
     publisher = {mathdoc},
     volume = {12},
     number = {9},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1976_12_9_a0/}
}