@article{SM_2006_197_9_a5,
     author = {V. L. Levin},
     title = {Best approximation problems},
     journal = {Sbornik. Mathematics},
     pages = {1353--1364},
     year = {2006},
     volume = {197},
     number = {9},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2006_197_9_a5/}
}