Abstract
Let f be a real continuous 2π-periodic function changing its sign in the fixed distinct points y i ∈ Y:= {y i } i∈ℤ such that for x ∈ [y i , y i−1], f(x) ≧ 0 if i is odd and f(x) ≦ 0 if i is even. Then for each n ≧ N(Y) we construct a trigonometric polynomial P n of order ≦ n, changing its sign at the same points y i ∈ Y as f, and
where N(Y) is a constant depending only on Y, c(s) is a constant depending only on s, ω 3(f, t) is the third modulus of smoothness of f and ∥ · ∥ is the max-norm.
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This work was done while the first author was visiting CPT-CNRS, Luminy, France, in June 2006.
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Dzyubenko, G.A., Gilewicz, J. Copositive approximation of periodic functions. Acta Math Hung 120, 301–314 (2008). https://doi.org/10.1007/s10474-008-6204-0
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DOI: https://doi.org/10.1007/s10474-008-6204-0