Σ-finite

In mathematics, a positive (or signed) measure μ defined on a σ-algebra Σ of subsets of a set X is called finite if μ(X) is a finite real number (rather than ∞). The measure μ is called σ-finite if X is the countable union of measurable sets with finite measure. A set in a measure space is said to have σ-finite measure if it is a countable union of measurable sets with finite measure.

A different but related notion that should not be confused with sigma-finiteness is s-finiteness

Let ${\displaystyle (X,{\mathcal {A}})}$ be a measurable space and ${\displaystyle \mu }$ a measure on it.

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