@article{SM_2003_194_5_a4,
     author = {Yu. B. Orochko},
     title = {Impenetrability condition for a~degenerate point of a~one-term symmetric differential operator of even order},
     journal = {Sbornik. Mathematics},
     pages = {745--774},
     year = {2003},
     volume = {194},
     number = {5},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2003_194_5_a4/}
}