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On the Bernstein conjecture in approximation theory. (English) Zbl 0648.41013

With \(E_{2n}(| x|)\) denoting the error of best uniform approximation to \(| x|\) by polynomials of degree at most 2n on the interval \([-1,+1]\), the famous Russian mathematician S. Bernstein in 1914 established the existence of a positive constant \(\beta\) for which \(\lim_{n\to \infty}2nE_{2n}(| x|)=:\beta.\) Moreover, by means of numerical calculations, Bernstein determined, in the same paper, the following upper and lower bounds for \(\beta\) : \(0.278<\beta <0.286\). Now, the average of these bounds is 0.282, which, as Bernstein noted as a “curious coincidence,” is very close to 1/(2\(\sqrt{\pi})=0.2820947917..\). This observation has over the years become known as the Bernstein Conjecture: Is \(\beta =1/(2\sqrt{\pi})?\) We show here that the Bernstein conjecture is false. In addition, we determine rigorous upper and lower bounds for \(\beta\), and by means of the Richardson extrapolation procedure, estimate \(\beta\) to approximately 50 decimal places.

MSC:

41A50 Best approximation, Chebyshev systems
41A25 Rate of convergence, degree of approximation
41A10 Approximation by polynomials

Software:

Algorithm 524
Full Text: DOI

Online Encyclopedia of Integer Sequences:

Decimal expansion of Bernstein’s constant.

References:

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