@article{DAN_1957_115_2_a7,
     author = {D. P. Kostomarov},
     title = {On boundary problems for eigen-values and characteristic functions of ordinary differential equations 
},
     journal = {Doklady Akademii Nauk},
     pages = {230--233},
     publisher = {mathdoc},
     volume = {115},
     number = {2},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_115_2_a7/}
}