Geodesic
On the theory of approximation of functions by linear combinations of Sturm–Liouville characteristic functions
Doklady Akademii Nauk, Tome 114 (1957) no. 2, pp. 263-266 
G. I. Natanson. On the theory of approximation of functions by linear combinations of Sturm–Liouville characteristic functions. Doklady Akademii Nauk, Tome 114 (1957) no. 2, pp. 263-266. http://geodesic.mathdoc.fr/item/DAN_1957_114_2_a7/
@article{DAN_1957_114_2_a7,
     author = {G. I. Natanson},
     title = {On the theory of approximation of functions by linear combinations of {Sturm{\textendash}Liouville} characteristic functions},
     journal = {Doklady Akademii Nauk},
     pages = {263--266},
     year = {1957},
     volume = {114},
     number = {2},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_114_2_a7/}
}
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