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Difference between revisions of "Dirichlet-function"

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The function which is equal to one at the rational points and to zero at the irrational points. It is also defined by the formula:
 
The function which is equal to one at the rational points and to zero at the irrational points. It is also defined by the formula:
  
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$$f(x)=\lim_{m\to\infty}\lim_{n\to\infty}(\cos m!\pi x)^{2n},$$
  
 
and belongs to the second Baire class (cf. [[Baire classes|Baire classes]]). It is not Riemann integrable on any segment but, since it is equal to zero almost-everywhere, it is Lebesgue integrable.
 
and belongs to the second Baire class (cf. [[Baire classes|Baire classes]]). It is not Riemann integrable on any segment but, since it is equal to zero almost-everywhere, it is Lebesgue integrable.

Revision as of 14:24, 28 August 2014

The function which is equal to one at the rational points and to zero at the irrational points. It is also defined by the formula:

$$f(x)=\lim_{m\to\infty}\lim_{n\to\infty}(\cos m!\pi x)^{2n},$$

and belongs to the second Baire class (cf. Baire classes). It is not Riemann integrable on any segment but, since it is equal to zero almost-everywhere, it is Lebesgue integrable.

References

[1] I.P. Natanson, "Theorie der Funktionen einer reellen Veränderlichen" , H. Deutsch , Frankfurt a.M. (1961) (Translated from Russian)
How to Cite This Entry:
Dirichlet-function. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Dirichlet-function&oldid=12889
This article was adapted from an original article by L.D. Kudryavtsev (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article