\Bibitem{Ber81} \by V.~I.~Berdyshev \paper Variation of the norms in the problem of best approximation \jour Mat. Zametki \yr 1981 \vol 29 \issue 2 \pages 181--196 \mathnet{http://mi.mathnet.ru/mzm6296} \mathscinet{http://mathscinet.ams.org/mathscinet-getitem?mr=610577} \zmath{https://zbmath.org/?q=an:0506.41027|0459.41016} \transl \jour Math. Notes \yr 1981 \vol 29 \issue 2 \pages 95--103 \crossref{https://doi.org/10.1007/BF01140919} \isi{https://gateway.webofknowledge.com/gateway/Gateway.cgi?GWVersion=2&SrcApp=Publons&SrcAuth=Publons_CEL&DestLinkType=FullRecord&DestApp=WOS_CPL&KeyUT=A1981MH74800018}