@article{DAN_1989_309_6_a5,
     author = {Yu. V. Pokorny and O. M. Penkin},
     title = {Sturm theorems for equations on graphs},
     journal = {Doklady Akademii Nauk},
     pages = {1306--1308},
     publisher = {mathdoc},
     volume = {309},
     number = {6},
     year = {1989},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1989_309_6_a5/}
}