# Difference between revisions of "Markov criterion"

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$$ | $$ | ||

P _ {n} ^ {*} ( x) = \ | P _ {n} ^ {*} ( x) = \ | ||

− | \sum _ { k= } | + | \sum _ { k= 1} ^ { n } |

c _ {k} ^ {*} \phi _ {k} ( x) | c _ {k} ^ {*} \phi _ {k} ( x) | ||

$$ | $$ | ||

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\int\limits _ { a } ^ { b } \left | | \int\limits _ { a } ^ { b } \left | | ||

f ( x) - | f ( x) - | ||

− | \sum _ { k= } | + | \sum _ { k= 1} ^ { n } c _ {k} \phi _ {k} ( x) \ |

\right | d x = | \right | d x = | ||

$$ | $$ |

## Revision as of 19:58, 17 December 2020

*for best integral approximation*

A theorem which in some cases enables one to give effectively the polynomial and the error of best integral approximation of a function $ f $. It was established by A.A. Markov in 1898 (see [1]). Let $ \{ \phi _ {k} ( x) \} $, $ k = 1 \dots n $, be a system of linearly independent functions continuous on the interval $ [ a , b ] $, and let the continuous function $ \psi $ change sign at the points $ x _ {1} < \dots < x _ {r} $ in $ ( a , b ) $ and be such that

$$ \int\limits _ { a } ^ { b } \phi _ {k} ( x) \mathop{\rm sgn} \psi ( x) d x = 0 ,\ \ k = 1 \dots n . $$

If the polynomial

$$ P _ {n} ^ {*} ( x) = \ \sum _ { k= 1} ^ { n } c _ {k} ^ {*} \phi _ {k} ( x) $$

has the property that the difference $ f - P _ {n} ^ {*} $ changes sign at the points $ x _ {1} \dots x _ {r} $, and only at those points, then $ P _ {n} ^ {*} $ is the polynomial of best integral approximation to $ f $ and

$$ \inf _ {\{ c _ {k} \} } \ \int\limits _ { a } ^ { b } \left | f ( x) - \sum _ { k= 1} ^ { n } c _ {k} \phi _ {k} ( x) \ \right | d x = $$

$$ = \ \int\limits _ { a } ^ { b } \left | f ( x) - P _ {n} ^ {*} ( x) \right | d x = \left | \int\limits _ { a } ^ { b } f ( x) \mathop{\rm sgn} \psi ( x) d x \right | . $$

For the system $ \{ 1 , \cos x \dots \cos n x \} $ on $ [ 0 , \pi ] $, $ \psi $ can be taken to be $ \cos ( n + 1) x $; for the system $ \{ \sin x \dots \sin n x \} $, $ 0 \leq x \leq \pi $, $ \psi $ can be taken to be $ \sin ( n + 1 ) x $; and for the system $ \{ 1 , x \dots x ^ {n} \} $, $ - 1 \leq x \leq 1 $, one can take $ \psi ( x) = \sin ( ( n + 2 ) \mathop{\rm arc} \cos x ) $.

#### References

[1] | A.A. Markov, "Selected works" , Moscow-Leningrad (1948) (In Russian) |

[2] | N.I. [N.I. Akhiezer] Achiezer, "Theory of approximation" , F. Ungar (1956) (Translated from Russian) |

[3] | I.K. Daugavet, "Introduction to the theory of approximation of functions" , Leningrad (1977) (In Russian) |

#### Comments

#### References

[a1] | E.W. Cheney, "Introduction to approximation theory" , Chelsea, reprint (1982) |

[a2] | M.W. Müller, "Approximationstheorie" , Akad. Verlagsgesellschaft (1978) |

[a3] | J.R. Rice, "The approximation of functions" , 1. Linear theory , Addison-Wesley (1964) |

**How to Cite This Entry:**

Markov criterion.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Markov_criterion&oldid=47770