Abstract
Suppose that a continuous 2π-periodic function f on the real axis ℝ changes its monotonicity at different ordered fixed points y i ∈ [− π, π), i = 1, …, 2s, s ∈ ℕ. In other words, there is a set Y:= {y i } i∈ℤ of points y i = y i+2s + 2π on ℝ such that, on [y i , y i−1], f is nondecreasing if i is odd and nonincreasing if i is even. For each n ≥ N(Y), we construct a trigonometric polynomial P n of order ≤ n changing its monotonicity at the same points y i ∈ Y as f and such that
where N(Y) is a constant depending only on Y, c(s) is a constant depending only on s, ω 2(f, ·) is the modulus of continuity of second order of the function f, and ∥ · ∥ is the max-norm.
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Original Russian Text © G. A. Dzyubenko, M. G. Pleshakov, 2008, published in Matematicheskie Zametki, 2008, Vol. 83, No. 2, pp. 199–209.
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Dzyubenko, G.A., Pleshakov, M.G. Comonotone approximation of periodic functions. Math Notes 83, 180–189 (2008). https://doi.org/10.1134/S0001434608010203
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DOI: https://doi.org/10.1134/S0001434608010203