Geodesic
On the value of the widths of some classes of functions from $L_{2}$
Dalʹnevostočnyj matematičeskij žurnal, Tome 21 (2021) no. 1, pp. 61-70.

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In this paper we find sharp inequalities of Jackson-Stechkin type between the best approximations of periodic differentiable functions by trigonometric polynomials and generalized moduli of continuity of $m$-th order in the space $L_{2}.$ The exact values of various $n$-widths of classes of functions from $L_{2}$ defined by the generalized modus of continuity of the $r$-th derivative of the function f are calculated. 
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M. R. Langarshoev. On the value of the widths of some classes of functions from $L_{2}$. Dalʹnevostočnyj matematičeskij žurnal, Tome 21 (2021) no. 1, pp. 61-70. http://geodesic.mathdoc.fr/item/DVMG_2021_21_1_a5/

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