In mathematics, specifically measure theory, 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 infinity if the subset is infinite.[1]
The counting measure can be defined on any measurable space (that is, any set along with a sigma-algebra) but is mostly used on countable sets.[1]
In formal notation, we can turn any set into a measurable space by taking the power set of as the sigma-algebra that is, all subsets of are measurable sets. Then the counting measure on this measurable space is the positive measure defined by
The counting measure on is σ-finite if and only if the space is countable.[3]
Take the measure space , where is the set of all subsets of the naturals and the counting measure. Take any measurable . As it is defined on , can be represented pointwise as
Each is measurable. Moreover . Still further, as each is a simple function
The counting measure is a special case of a more general construction. With the notation as above, any function defines a measure on via