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Properties of algebraic, trigonometric or generalized polynomials that distinguish them as solutions of some extremal problems.
 
Properties of algebraic, trigonometric or generalized polynomials that distinguish them as solutions of some extremal problems.
  
For example, the [[Chebyshev polynomials|Chebyshev polynomials]] <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e0372201.png" /> have minimal norm in the space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e0372202.png" /> among all algebraic polynomials of degree <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e0372203.png" /> with leading coefficient <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e0372204.png" /> (P.L. Chebychev, 1853); therefore they are the solution of the extremal problem
+
For example, the [[Chebyshev polynomials|Chebyshev polynomials]] $  T _ {n} ( x) = \cos ( n  { \mathop{\rm arc}  \cos }  x ) = 2  ^ {n-} 1 x  ^ {n} + \dots $
 +
have minimal norm in the space $  C ( [ - 1 , 1 ] ) $
 +
among all algebraic polynomials of degree $  n $
 +
with leading coefficient $  2  ^ {n-} 1 $(
 +
P.L. Chebychev, 1853); therefore they are the solution of the extremal problem
  
<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/e/e037/e037220/e0372205.png" /></td> </tr></table>
+
$$
 +
\max _ {x \in [ - 1 , 1 ] }  | 2  ^ {n-} 1 x  ^ {n} +
 +
a _ {1} x  ^ {n-} 1 + \dots + a _ {n} |  \rightarrow  \inf _
 +
{a = ( a _ {1} \dots a _ {n} ) } .
 +
$$
  
In other words, the polynomial <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e0372206.png" /> differs least from zero in the space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e0372207.png" /> among all polynomials of degree <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e0372208.png" /> with leading coefficient equal to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e0372209.png" />.
+
In other words, the polynomial $  T _ {n} $
 +
differs least from zero in the space $  C ( [ - 1 , 1 ] ) $
 +
among all polynomials of degree $  n $
 +
with leading coefficient equal to $  2  ^ {n-} 1 $.
  
Extremal problems in spaces of polynomials are mainly studied in the spaces <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722010.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722011.png" />. In this context most of the available results are connected with the cases <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722012.png" />, 2 and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722013.png" /> (the metric of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722014.png" />). In particular, these metrics are used to find the explicit form of the polynomials that differ least from zero. In the metric of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722015.png" /> these are the Chebychev polynomials of the second kind, in the metric of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722016.png" /> one obtains the [[Legendre polynomials|Legendre polynomials]]; concerning the metric of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722017.png" /> see above. The set of classical orthogonal polynomials deviating least from zero in a weighted <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722018.png" />-space ([[Laguerre polynomials|Laguerre polynomials]]; [[Hermite polynomials|Hermite polynomials]]; [[Jacobi polynomials|Jacobi polynomials]]; etc.) has also been described.
+
Extremal problems in spaces of polynomials are mainly studied in the spaces $  L _ {p} ( [ a , b ] ) $,  
 +
$  1 \leq  p \leq  \infty $.  
 +
In this context most of the available results are connected with the cases $  p = 1 $,  
 +
2 and $  \infty $(
 +
the metric of $  C $).  
 +
In particular, these metrics are used to find the explicit form of the polynomials that differ least from zero. In the metric of $  L _ {1} $
 +
these are the Chebychev polynomials of the second kind, in the metric of $  L _ {2} $
 +
one obtains the [[Legendre polynomials|Legendre polynomials]]; concerning the metric of $  C $
 +
see above. The set of classical orthogonal polynomials deviating least from zero in a weighted $  L _ {2} $-
 +
space ([[Laguerre polynomials|Laguerre polynomials]]; [[Hermite polynomials|Hermite polynomials]]; [[Jacobi polynomials|Jacobi polynomials]]; etc.) has also been described.
  
E.I. Zolotarev (1877) considered the question of determining polynomials of the form <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722019.png" /> (with two fixed leading coefficients) that deviate least from zero in the metric of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722020.png" />. He found a one-parameter family of polynomials that solved this problem, and expressed them in terms of elliptic functions.
+
E.I. Zolotarev (1877) considered the question of determining polynomials of the form $  x  ^ {n} + \sigma x  ^ {n-} 1 + a _ {2} x  ^ {n-} 2 + \dots + a _ {n} $(
 +
with two fixed leading coefficients) that deviate least from zero in the metric of $  C $.  
 +
He found a one-parameter family of polynomials that solved this problem, and expressed them in terms of elliptic functions.
  
The Chebychev polynomials are extremal in the problem of an inequality for the derivatives; namely, the exact [[Markov inequality|Markov inequality]] (where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722021.png" /> is a polynomial of degree <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722022.png" />)
+
The Chebychev polynomials are extremal in the problem of an inequality for the derivatives; namely, the exact [[Markov inequality|Markov inequality]] (where $  P _ {n} $
 +
is a polynomial of degree $  \leq  n $)
  
<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/e/e037/e037220/e03722023.png" /></td> <td valign="top" style="width:5%;text-align:right;">(*)</td></tr></table>
+
$$ \tag{* }
 +
\| P _ {n}  ^ {(} k) \| _ {C ( [ - 1 , 1 ] ) }
 +
\leq  | T _ {n}  ^ {(} k) ( 1) | \cdot \| P _ {n} \| _ {C ( [ - 1 , 1 ] ) }
 +
$$
  
holds, with equality for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722024.png" />. Inequality (*) was proved by A.A. Markov (1889) for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722025.png" />, and by V.A. Markov (1892) for all other values of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/e/e037/e037220/e03722026.png" />. Concerning a similar inequality for trigonometric polynomials see [[Bernstein inequality|Bernstein inequality]].
+
holds, with equality for $  T _ {n} $.  
 +
Inequality (*) was proved by A.A. Markov (1889) for $  k = 1 $,  
 +
and by V.A. Markov (1892) for all other values of $  k $.  
 +
Concerning a similar inequality for trigonometric polynomials see [[Bernstein inequality|Bernstein inequality]].
  
 
Some extremal properties of algebraic and trigonometric polynomials in the uniform metric carry over to Chebychev systems of functions (see [[#References|[2]]]). Concerning the theory of extremal problems and extremal properties of polynomials see [[#References|[6]]].
 
Some extremal properties of algebraic and trigonometric polynomials in the uniform metric carry over to Chebychev systems of functions (see [[#References|[2]]]). Concerning the theory of extremal problems and extremal properties of polynomials see [[#References|[6]]].
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====References====
 
====References====
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  P.L. Chebychev,  "Oeuvres" , '''1–2''' , Chelsea, reprint  (No date)</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  S.N. Bernshtein,  "Leçons sur les propriétés extrémales de la meilleure approximation des fonctions analytiques d'une variable réelle" , Chelsea, reprint  (1970)  (Translated from Russian)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  V.L. Goncharov,  "The theory of interpolation and approximation of functions" , Moscow  (1954)  (In Russian)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  N.I. [N.I. Akhiezer] Achiezer,  "Theory of approximation" , F. Ungar  (1956)  (Translated from Russian)</TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top">  E.V. Voronovskaya,  "The functional method and its applications" , Amer. Math. Soc.  (1970)  (Translated from Russian)</TD></TR><TR><TD valign="top">[6]</TD> <TD valign="top">  V.M. Tikhomirov,  "Some problems in approximation theory" , Moscow  (1976)  (In Russian)</TD></TR></table>
 
<table><TR><TD valign="top">[1]</TD> <TD valign="top">  P.L. Chebychev,  "Oeuvres" , '''1–2''' , Chelsea, reprint  (No date)</TD></TR><TR><TD valign="top">[2]</TD> <TD valign="top">  S.N. Bernshtein,  "Leçons sur les propriétés extrémales de la meilleure approximation des fonctions analytiques d'une variable réelle" , Chelsea, reprint  (1970)  (Translated from Russian)</TD></TR><TR><TD valign="top">[3]</TD> <TD valign="top">  V.L. Goncharov,  "The theory of interpolation and approximation of functions" , Moscow  (1954)  (In Russian)</TD></TR><TR><TD valign="top">[4]</TD> <TD valign="top">  N.I. [N.I. Akhiezer] Achiezer,  "Theory of approximation" , F. Ungar  (1956)  (Translated from Russian)</TD></TR><TR><TD valign="top">[5]</TD> <TD valign="top">  E.V. Voronovskaya,  "The functional method and its applications" , Amer. Math. Soc.  (1970)  (Translated from Russian)</TD></TR><TR><TD valign="top">[6]</TD> <TD valign="top">  V.M. Tikhomirov,  "Some problems in approximation theory" , Moscow  (1976)  (In Russian)</TD></TR></table>
 
 
  
 
====Comments====
 
====Comments====
 
  
 
====References====
 
====References====
 
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  E.W. Cheney,  "Introduction to approximation theory" , Chelsea, reprint  (1982)</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  I.P. Natanson,  "Constructive function theory" , '''1–3''' , F. Ungar  (1964–1965)  (Translated from Russian)</TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  T.J. Rivlin,  "The Chebyshev polynomials" , Wiley  (1974)</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  H.S. Shapiro,  "Topics in approximation theory" , Springer  (1971)</TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  G.G. Lorentz,  "Approximation of functions" , Holt, Rinehart &amp; Winston  (1966)</TD></TR></table>
 
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  E.W. Cheney,  "Introduction to approximation theory" , Chelsea, reprint  (1982)</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  I.P. Natanson,  "Constructive function theory" , '''1–3''' , F. Ungar  (1964–1965)  (Translated from Russian)</TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  T.J. Rivlin,  "The Chebyshev polynomials" , Wiley  (1974)</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  H.S. Shapiro,  "Topics in approximation theory" , Springer  (1971)</TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  G.G. Lorentz,  "Approximation of functions" , Holt, Rinehart &amp; Winston  (1966)</TD></TR></table>

Latest revision as of 19:38, 5 June 2020


Properties of algebraic, trigonometric or generalized polynomials that distinguish them as solutions of some extremal problems.

For example, the Chebyshev polynomials $ T _ {n} ( x) = \cos ( n { \mathop{\rm arc} \cos } x ) = 2 ^ {n-} 1 x ^ {n} + \dots $ have minimal norm in the space $ C ( [ - 1 , 1 ] ) $ among all algebraic polynomials of degree $ n $ with leading coefficient $ 2 ^ {n-} 1 $( P.L. Chebychev, 1853); therefore they are the solution of the extremal problem

$$ \max _ {x \in [ - 1 , 1 ] } | 2 ^ {n-} 1 x ^ {n} + a _ {1} x ^ {n-} 1 + \dots + a _ {n} | \rightarrow \inf _ {a = ( a _ {1} \dots a _ {n} ) } . $$

In other words, the polynomial $ T _ {n} $ differs least from zero in the space $ C ( [ - 1 , 1 ] ) $ among all polynomials of degree $ n $ with leading coefficient equal to $ 2 ^ {n-} 1 $.

Extremal problems in spaces of polynomials are mainly studied in the spaces $ L _ {p} ( [ a , b ] ) $, $ 1 \leq p \leq \infty $. In this context most of the available results are connected with the cases $ p = 1 $, 2 and $ \infty $( the metric of $ C $). In particular, these metrics are used to find the explicit form of the polynomials that differ least from zero. In the metric of $ L _ {1} $ these are the Chebychev polynomials of the second kind, in the metric of $ L _ {2} $ one obtains the Legendre polynomials; concerning the metric of $ C $ see above. The set of classical orthogonal polynomials deviating least from zero in a weighted $ L _ {2} $- space (Laguerre polynomials; Hermite polynomials; Jacobi polynomials; etc.) has also been described.

E.I. Zolotarev (1877) considered the question of determining polynomials of the form $ x ^ {n} + \sigma x ^ {n-} 1 + a _ {2} x ^ {n-} 2 + \dots + a _ {n} $( with two fixed leading coefficients) that deviate least from zero in the metric of $ C $. He found a one-parameter family of polynomials that solved this problem, and expressed them in terms of elliptic functions.

The Chebychev polynomials are extremal in the problem of an inequality for the derivatives; namely, the exact Markov inequality (where $ P _ {n} $ is a polynomial of degree $ \leq n $)

$$ \tag{* } \| P _ {n} ^ {(} k) \| _ {C ( [ - 1 , 1 ] ) } \leq | T _ {n} ^ {(} k) ( 1) | \cdot \| P _ {n} \| _ {C ( [ - 1 , 1 ] ) } $$

holds, with equality for $ T _ {n} $. Inequality (*) was proved by A.A. Markov (1889) for $ k = 1 $, and by V.A. Markov (1892) for all other values of $ k $. Concerning a similar inequality for trigonometric polynomials see Bernstein inequality.

Some extremal properties of algebraic and trigonometric polynomials in the uniform metric carry over to Chebychev systems of functions (see [2]). Concerning the theory of extremal problems and extremal properties of polynomials see [6].

References

[1] P.L. Chebychev, "Oeuvres" , 1–2 , Chelsea, reprint (No date)
[2] S.N. Bernshtein, "Leçons sur les propriétés extrémales de la meilleure approximation des fonctions analytiques d'une variable réelle" , Chelsea, reprint (1970) (Translated from Russian)
[3] V.L. Goncharov, "The theory of interpolation and approximation of functions" , Moscow (1954) (In Russian)
[4] N.I. [N.I. Akhiezer] Achiezer, "Theory of approximation" , F. Ungar (1956) (Translated from Russian)
[5] E.V. Voronovskaya, "The functional method and its applications" , Amer. Math. Soc. (1970) (Translated from Russian)
[6] V.M. Tikhomirov, "Some problems in approximation theory" , Moscow (1976) (In Russian)

Comments

References

[a1] E.W. Cheney, "Introduction to approximation theory" , Chelsea, reprint (1982)
[a2] I.P. Natanson, "Constructive function theory" , 1–3 , F. Ungar (1964–1965) (Translated from Russian)
[a3] T.J. Rivlin, "The Chebyshev polynomials" , Wiley (1974)
[a4] H.S. Shapiro, "Topics in approximation theory" , Springer (1971)
[a5] G.G. Lorentz, "Approximation of functions" , Holt, Rinehart & Winston (1966)
How to Cite This Entry:
Extremal properties of polynomials. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Extremal_properties_of_polynomials&oldid=46894
This article was adapted from an original article by V.M. Tikhomirov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article