In
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
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
for all
where
denotes the
cardinality of the set
[2]
The counting measure on is
σ-finite
if and only if the space
is
Integration on with counting measure
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
Hence by the monotone convergence theorem
Discussion
The counting measure is a special case of a more general construction. With the notation as above, any function defines a measure on via
where the possibly uncountable sum of real numbers is defined to be the
supremum
of the sums over all finite subsets, that is,
Taking
for all
gives the counting measure.
See also
References