# Counting measure

In madematics, de counting measure is an intuitive way to put a measure on any set: de "size" of a subset is taken to be: de number of ewements in de subset if de subset has finitewy many ewements, and if de subset is infinite.[1]

The counting measure can be defined on any measurabwe set, but is mostwy used on countabwe sets.[1]

In formaw notation, we can make any set X into a measurabwe space by taking de sigma-awgebra ${\dispwaystywe \Sigma }$ of measurabwe subsets to consist of aww subsets of ${\dispwaystywe X}$. Then de counting measure ${\dispwaystywe \mu }$ on dis measurabwe space ${\dispwaystywe (X,\Sigma )}$ is de positive measure ${\dispwaystywe \Sigma \rightarrow [0,+\infty ]}$ defined by

${\dispwaystywe \mu (A)={\begin{cases}\vert A\vert &{\text{if }}A{\text{ is finite}}\\+\infty &{\text{if }}A{\text{ is infinite}}\end{cases}}}$

for aww ${\dispwaystywe A\in \Sigma }$, where ${\dispwaystywe \vert A\vert }$ denotes de cardinawity of de set ${\dispwaystywe A}$.[2]

The counting measure on ${\dispwaystywe (X,\Sigma )}$ is σ-finite if and onwy if de space ${\dispwaystywe X}$ is countabwe.[3]

## Discussion

The counting measure is a speciaw case of a more generaw construct. Wif de notation as above, any function ${\dispwaystywe f\cowon X\to [0,\infty )}$ defines a measure ${\dispwaystywe \mu }$ on ${\dispwaystywe (X,\Sigma )}$ via

${\dispwaystywe \mu (A):=\sum _{a\in A}f(a)\,\foraww A\subseteq X,}$

where de possibwy uncountabwe sum of reaw numbers is defined to be de sup of de sums over aww finite subsets, i.e.,

${\dispwaystywe \sum _{y\in Y\subseteq \madbb {R} }y:=\sup _{F\subseteq Y,|F|<\infty }\weft\{\sum _{y\in F}y\right\}.}$

Taking f(x)=1 for aww x in X produces de counting measure.

## Notes

1. ^ a b
2. ^ Schiwwing (2005), p.27
3. ^ Hansen (2009) p.47

## References

• Schiwwing, René L. (2005). Measures, Integraw and Martingawes. Cambridge University Press.
• Hansen, Ernst (2009). Measure Theory, Fourf Edition, uh-hah-hah-hah. Department of Madematicaw Science, University of Copenhagen, uh-hah-hah-hah.
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