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4. Berens, H., Lorentz, G.G.: Inverse theorems for Bernstein polynomials. Indiana Univ. Math. J.21, 693?708 (1972)
5. Butzer, P.L., Nessel, R.J.: Fourier Analysis and Approximation, Vol. I: One-Dimensional Theory. Basel: Birkhäuser and New York: Academic Press 1971