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Difference between revisions of "Alternation, points of"

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A sequence of points
 
A sequence of points
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a0120701.png" /></td> </tr></table>
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$$\{x_i\}_0^{n+1}\subset Q,\quad a\leq x_0<x_1<\ldots<x_{n+1}\leq b,$$
  
at which the difference <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a0120702.png" /> assumes non-zero values of alternating signs. Here <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a0120703.png" /> is a continuous function on the closed set <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a0120704.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a0120705.png" /> is an algebraic polynomial of degree not exceeding <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a0120706.png" />. In a similar manner the concept of points of alternation is introduced for polynomials in a Chebyshev system of functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a0120707.png" /> (which satisfy the [[Haar condition|Haar condition]]). If, in this situation, all absolute values of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a0120708.png" /> are equal to
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at which the difference $\alpha_i=f(x_i)-P_n(x_i)$ assumes non-zero values of alternating signs. Here $f(x)$ is a continuous function on the closed set $Q\subset[a,b]$ and $P_n(x)$ is an algebraic polynomial of degree not exceeding $n$. In a similar manner the concept of points of alternation is introduced for polynomials in a Chebyshev system of functions $\{s_k(x)\}_0^n$ (which satisfy the [[Haar condition|Haar condition]]). If, in this situation, all absolute values of $\alpha_i$ are equal to
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a0120709.png" /></td> </tr></table>
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$$\max_{x\in Q}|f(x)-P_n(x)|,$$
  
then the points <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/a/a012/a012070/a01207010.png" /> are called Chebyshev points of alternation. Points of alternation play an important role in the theory of approximation of functions. E.g., the [[De la Vallée-Poussin theorem|de la Vallée-Poussin theorem]] (the alternation theorem) and the Chebyshev criterion (cf. [[Chebyshev alternation|Chebyshev alternation]]) are formulated in terms of points of alternation. Points of alternation are also employed in constructing polynomials of best approximation.
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then the points $\{x_i\}_0^{n+1}$ are called Chebyshev points of alternation. Points of alternation play an important role in the theory of approximation of functions. E.g., the [[De la Vallée-Poussin theorem|de la Vallée-Poussin theorem]] (the alternation theorem) and the Chebyshev criterion (cf. [[Chebyshev alternation|Chebyshev alternation]]) are formulated in terms of points of alternation. Points of alternation are also employed in constructing polynomials of best approximation.
  
  

Latest revision as of 18:38, 26 September 2014

A sequence of points

$$\{x_i\}_0^{n+1}\subset Q,\quad a\leq x_0<x_1<\ldots<x_{n+1}\leq b,$$

at which the difference $\alpha_i=f(x_i)-P_n(x_i)$ assumes non-zero values of alternating signs. Here $f(x)$ is a continuous function on the closed set $Q\subset[a,b]$ and $P_n(x)$ is an algebraic polynomial of degree not exceeding $n$. In a similar manner the concept of points of alternation is introduced for polynomials in a Chebyshev system of functions $\{s_k(x)\}_0^n$ (which satisfy the Haar condition). If, in this situation, all absolute values of $\alpha_i$ are equal to

$$\max_{x\in Q}|f(x)-P_n(x)|,$$

then the points $\{x_i\}_0^{n+1}$ are called Chebyshev points of alternation. Points of alternation play an important role in the theory of approximation of functions. E.g., the de la Vallée-Poussin theorem (the alternation theorem) and the Chebyshev criterion (cf. Chebyshev alternation) are formulated in terms of points of alternation. Points of alternation are also employed in constructing polynomials of best approximation.


Comments

A sequence of Chebyshev points of alternation is also called an alternating set [a1], Chapt. 1.

References

[a1] T.J. Rivlin, "An introduction to the approximation of functions" , Dover, reprint (1981)
[a2] M.W. Müller, "Approximationstheorie" , Akad. Verlagsgesellschaft (1978)
[a3] G.W. Meinardus, "Approximation von Funktionen und ihre numerische Behandlung" , Springer (1964)
[a4] E.W. Cheney, "Introduction to approximation theory" , Chelsea, reprint (1982) pp. 203ff
How to Cite This Entry:
Alternation, points of. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Alternation,_points_of&oldid=33400
This article was adapted from an original article by Yu.N. Subbotin (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article