In the case where a function f continuous on a segment changes its sign at s points y i : − 1 < y s < y s−1 < ... < y 1 < 1, for any n ∈ ℕ greater than a certain constant N(k, y i ) that depends only on k ∈ ℕ and \( \underset{i=1,\dots, s-1}{\min}\left\{{y}_i-{y}_{i+1}\right\}, \) we determine an algebraic polynomial P n of degree ≤ n such that: P n has the same sign as f everywhere except, possibly, small neighborhoods of the points \( {y}_i:\left({y}_i-{\rho}_n\left({y}_i\right),{y}_i+{\rho}_n\left({y}_i\right)\right),\kern1em {\rho}_n(x):= 1/{n}^2+\sqrt{1-{x}^2}/n,\kern1em {P}_n\left({y}_i\right)=0, \) and |f(x) − P n (x)| ≤ c(k, s)ω k (f, ρ n (x)), x ∈ [−1, 1], where c(k, s) is a constant that depends only on k and s and ω k (f, ·) is the modulus of continuity of the function f of order k.
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Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 69, No. 5, pp. 641–649, May, 2017.
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Dzyubenko, H.A. Pointwise Estimation of the Almost Copositive Approximation of Continuous Functions by Algebraic Polynomials. Ukr Math J 69, 746–756 (2017). https://doi.org/10.1007/s11253-017-1392-9
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DOI: https://doi.org/10.1007/s11253-017-1392-9