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In [[mathematics]], the '''counting measure''' is an intuitive way to put a [[measure (mathematics)|measure]] on any [[Set (mathematics)|set]]: the "size" of a [[subset]] is taken to be the number of elements in the subset, if the subset has finitely many elements, and [[∞]] if the subset is [[infinite_set|infinite]].<ref name="pm">{{PlanetMath|urlname=CountingMeasure|title=Counting Measure}}</ref> | |||
The counting measure can be defined on any measurable set, but is mostly used on [[countable]] sets.<ref name="pm" /> | |||
In formal notation, we can make any set <math>X</math> into a [[measurable space]] by taking the [[sigma-algebra]] <math>\Sigma </math> of measurable subsets to consist of all subsets of <math>X</math>. Then the counting measure <math>\mu</math> on this measurable space <math>(X,\Sigma)</math> is the positive measure <math>\Sigma\rightarrow[0,+\infty]</math> defined by | |||
:<math> | |||
\mu(A)=\begin{cases} | |||
\vert A \vert & \text{if } A \text{ is finite}\\ | |||
+\infty & \text{if } A \text{ is infinite} | |||
\end{cases} | |||
</math> | |||
for all <math>A\in\Sigma</math>, where <math>\vert A\vert</math> denotes the [[cardinality]] of the set <math>A</math>.<ref>Schilling (2005), p.27</ref> | |||
The counting measure on <math>(X,\Sigma)</math> is [[σ-finite]] if and only if the space <math>X</math> is [[countable]].<ref>Hansen (2009) p.47</ref> | |||
==Notes== | |||
{{Reflist}} | |||
==References== | |||
*Schilling, René L. (2005)."Measures, Integral and Martingales". Cambridge University Press. | |||
*Hansen, Ernst (2009)."Measure theory, Fourth Edition". Department of Mathematical Science, University of Copenhagen. | |||
{{DEFAULTSORT:Counting Measure}} | |||
[[Category:Measures (measure theory)]] |
Revision as of 05:00, 4 February 2014
In mathematics, the counting measure is an intuitive way to put a measure on any set: the "size" of a subset is taken to be the number of elements in the subset, if the subset has finitely many elements, and ∞ if the subset is infinite.[1]
The counting measure can be defined on any measurable set, but is mostly used on countable sets.[1]
In formal notation, we can make any set into a measurable space by taking the sigma-algebra of measurable subsets to consist of all subsets of . Then the counting measure on this measurable space is the positive measure defined by
for all , where denotes the cardinality of the set .[2]
The counting measure on is σ-finite if and only if the space is countable.[3]
Notes
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References
- Schilling, René L. (2005)."Measures, Integral and Martingales". Cambridge University Press.
- Hansen, Ernst (2009)."Measure theory, Fourth Edition". Department of Mathematical Science, University of Copenhagen.
- ↑ 1.0 1.1 Template:PlanetMath
- ↑ Schilling (2005), p.27
- ↑ Hansen (2009) p.47