Numer Algor (2012) 59:119–129
DOI 10.1007/s11075-011-9479-0
ORIGINAL PAPER
New estimates in Voronovskaja’s theorem
Gancho Tachev
Received: 10 March 2011 / Accepted: 27 May 2011 /
Published online: 14 July 2011
© Springer Science+Business Media, LLC 2011
Abstract In the present article we establish pointwise variant of E. V.
Voronovskaja’s 1932 result, concerning the degree of approximation of
Bernstein operator, applied to functions f ∈ C
3
[0, 1].
Keywords Bernstein polynomials · Voronovskaja-type theorem ·
Moduli of continuity · Degree of approximation
1 Introduction
For every function f ∈ C[0, 1] the Bernstein polynomial operator is given by
B
n
( f ; x) =
n
k=0
f (
k
n
) ·
n
k
x
k
(1 − x)
n−k
, x ∈[0, 1]
For this operator the theorem of Voronovskaja was first proved in [20] and is
given in the book of DeVore and Lorentz [2] as follows:
Theorem A If f is bounded on [0, 1], dif ferentiable in some neighborhood of
x and has second derivative f
′′
(x) for some x ∈[0, 1], then
lim
n→∞
n ·[ B
n
( f, x) − f (x)]=
x(1 − x)
2
· f
′′
(x).
If f ∈ C
2
[0, 1], the convergence is uniform.
G. Tachev (B )
University of Architecture, 1 Hr.Smirnenski Blvd., Sofia, 1046, Bulgaria
e-mail: gtt_fte@uacg.bg