# Difference between revisions of "Bernstein polynomials"

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Algebraic polynomials defined by the formula

$$B _ {n} (f; x ) = \ B _ {n} (x ) =$$

$$= \ \sum _ { k=0 } ^ { n } f \left ( { \frac{k}{n} } \right ) \left ( \begin{array}{c} n \\ k \end{array} \right ) x ^ {k} (1-x) ^ {n-k} ,\ n = 1, 2 ,\dots .$$

Introduced by S.N. Bernshtein in 1912 (cf. ). The sequence of Bernstein polynomials converges uniformly to a function $f$ on the segment $0 \leq x \leq 1$ if $f$ is continuous on this segment. For a function which is bounded by $C$, $0 < C < 1$, with a discontinuity of the first kind,

$$B _ {n} (f; C) \rightarrow \ { \frac{f (C _ {-} ) + f (C _ {+} ) }{2} } .$$

The equation

$$B _ {n} (f; c) - f (c) = \ \frac{f ^ { \prime\prime } (c)c(1-c) }{2n} + o \left ( \frac{1}{n} \right )$$

is valid if $f$ is twice differentiable at the point $c$. If the $k$- th derivative $f ^ { (k) }$ of the function is continuous on the segment $0 \leq x \leq 1$, the convergence

$$B _ {n} ^ { (k) } (f; x) \rightarrow f ^ { (k) } (x)$$

is uniform on this segment. A study was made ([1b], [5]) of the convergence of Bernstein polynomials in the complex plane if $f$ is analytic on the segment $0 \leq x \leq 1$.

#### References

 [1a] S.N. Bernshtein, , Collected works , 1 , Moscow (1952) pp. 105–106 [1b] S.N. Bernshtein, , Collected works , 2 , Moscow (1954) pp. 310–348 [2] V.L. Goncharov, "The theory of interpolation and approximation of functions" , Moscow (1954) (In Russian) [3] V.A. Baskakov, "An instance of a sequence of linear positive operators in the space of continuous functions" Dokl. Akad. Nauk SSSR , 113 : 2 (1957) pp. 249–251 (In Russian) [4] P.P. Korovkin, "Linear operators and approximation theory" , Hindushtan Publ. Comp. (1960) (Translated from Russian) [5] L.V. Kantorovich, Izv. Akad. Nauk SSSR Ser. Mat. , 8 (1931) pp. 1103–1115

#### Comments

There is also a multi-variable generalization: generalized Bernstein polynomials defined by the completely analogous formula

$$B _ {\mathbf n } (f, x _ {1} \dots x _ {k} ) =$$

$$= \ \sum _ { i _ {1} = 0 } ^ { {n _ 1 } } \dots \sum _ {i _ {k} = 0 } ^ { {n _ k} } f \left ( \frac{i _ {1} }{n _ {1} } \dots \frac{i _ {k} }{n _ {k} } \right ) \ \left ( \begin{array}{c} n _ {1} \\ i _ {1} \end{array} \right ) \dots \left ( \begin{array}{c} n _ {k} \\ i _ {k} \end{array} \right ) \times$$

$$\times x _ {1} ^ {i _ {1} } (1 - x _ {1} ) ^ {n _ {1} - i _ {1} } \dots x _ {k} ^ {i _ {k} } (1 - x _ {k} ) ^ {n _ {k} - i _ {k} } .$$

Here $\mathbf n$ stands for the multi-index $\mathbf n = ( n _ {1} \dots n _ {k} )$.

As in the one variable case these provide explicit polynomial approximants for the more-variable Weierstrass approximation and Stone–Weierstrass theorems. For the behaviour of Bernstein polynomials in the complex plane and applications to movement problems, cf. also [a3].

#### References

 [a1] P.J. Davis, "Interpolation and approximation" , Dover, reprint (1975) pp. 108–126 [a2] T.J. Rivlin, "An introduction to the approximation of functions" , Dover, reprint (1981) [a3] G.G. Lorentz, "Bernstein polynomials" , Univ. Toronto Press (1953)
How to Cite This Entry:
Bernstein polynomials. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Bernstein_polynomials&oldid=13289
This article was adapted from an original article by P.P. Korovkin (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article