structural:summarization, quality:correction
Sigma-finite measures

A measure space (X, Σ, μ) {\displaystyle (X,\Sigma,\mu)} is called finite if μ (X) {\displaystyle \mu (X)} is a finite real number (rather than ∞ {\displaystyle \infty }). Nonzero finite measures are analogous to probability measures in the sense that any finite measure μ {\displaystyle \mu } is proportional to the probability measure 1 μ (X) μ. {\displaystyle {\frac {1}{\mu (X)}}\mu.} A measure μ {\displaystyle \mu } is called σ-finite if X {\displaystyle X} can be decomposed into a countable union of measurable sets of finite measure. Analogously, a set in a measure space is said to have a σ-finite measure if it is a countable union of sets with finite measure.

For example, the real numbers with the standard Lebesgue measure are σ-finite but not finite. Consider the closed intervals {\displaystyle } for all integers k; {\displaystyle k;} there are countably many such intervals, each has measure 1, and their union is the entire real line. Alternatively, consider the real numbers with the counting measure, which assigns to each finite set of reals the number of points in the set. This measure space is not σ-finite, because every set with finite measure contains only finitely many points, and it would take uncountably many such sets to cover the entire real line. The σ-finite measure spaces have some very convenient properties; σ-finiteness can be compared in this respect to the Lindelöf property of topological spaces. They can be also thought of as a vague generalization of the idea that a measure space may have 'uncountable measure'.