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.
The counting measure can be defined on any measurable set, but is mostly used on countable sets.
In formal notation, we can make any set X 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 .
The counting measure on is σ-finite if and only if the space is countable.
The counting measure is a special case of a more general construct. With the notation as above, any function defines a measure
where the possibly uncountable sum of real numbers is defined to be the sup of the sums over all finite subsets, i.e.,
Taking f(x)=1 for all x in X produces the counting measure.
- 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.