Abstract
We prove the existence of a function fcontinuous and convex on [−1, 1] and such that, for any sequence {p n} n= 1 ∞of algebraic polynomials p nof degree ≤ nconvex on [−1, 1], the following relation is true: \(\begin{array}{*{20}c} {\lim \sup } \\ {n \to \infty } \\ \end{array} \begin{array}{*{20}c} {\max } \\{x \in [ - 1,1]} \\ \end{array} \frac{{|f(x) - p_n (x)|}}{{\omega _4 (\rho _n (x),f)}} = \infty \), where ω4(t, f) is the fourth modulus of continuity of the function fand \(\rho _n \left( x \right): = \frac{1}{{n^2 }} + \frac{1}{n}\sqrt {1 - x^2 } \). We generalize this result to q-convex functions.
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Yushchenko, L.P. On One Counterexample in Convex Approximation. Ukrainian Mathematical Journal 52, 1956–1963 (2000). https://doi.org/10.1023/A:1010420313286
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DOI: https://doi.org/10.1023/A:1010420313286