@article{MMO_1968_18_a5,
     author = {M. B. Barban and P. P. Vekhov},
     title = {An extremal problem},
     journal = {Trudy Moskovskogo matemati\v{c}eskogo ob\^{s}estva},
     pages = {83--90},
     publisher = {mathdoc},
     volume = {18},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MMO_1968_18_a5/}
}