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In measure theory, a branch of mathematics that studies generalized notions of volumes, an **s-finite measure** is a special type of measure. An s-finite measure is more general than a finite measure, but allows one to generalize certain proofs for finite measures.

The s-finite measures should not be confused with the σ-finite (sigma-finite) measures.

Let be a measurable space and a measure on this measurable space. The measure is called an s-finite measure, if it can be written as a countable sum of finite measures (),^{[1]}

The Lebesgue measure is an s-finite measure. For this, set

and define the measures by

for all measurable sets . These measures are finite, since for all measurable sets , and by construction satisfy

Therefore the Lebesgue measure is s-finite.

Every σ-finite measure is s-finite, but not every s-finite measure is also σ-finite.

To show that every σ-finite measure is s-finite, let be σ-finite. Then there are measurable disjoint sets with and

Then the measures

are finite and their sum is . This approach is just like in the example above.

An example for an s-finite measure that is not σ-finite can be constructed on the set with the σ-algebra . For all , let be the counting measure on this measurable space and define

The measure is by construction s-finite (since the counting measure is finite on a set with one element). But is not σ-finite, since

So cannot be σ-finite.

For every s-finite measure , there exists an equivalent probability measure , meaning that .^{[1]} One possible equivalent probability measure is given by