@article{DE_1965_1_3_a4,
     author = {\`E. I. Grudo},
     title = {On an analytic theory of systems of linear differential equations with an irregular singular point},
     journal = {Differencialʹnye uravneni\^a},
     pages = {318--326},
     publisher = {mathdoc},
     volume = {1},
     number = {3},
     year = {1965},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1965_1_3_a4/}
}