Lebesgue outer measure
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extension theorem
Let be a subset of , let be the traditional definition of the length of an interval : If , then . Finally, let be the set consisting of the values
for all possible countable collections of open intervals that covers (that is, ). Then the Lebesgue outer measure of is defined by:
Note that is an outer measure space (http://planetmath.org/OuterMeasure2). In particular:
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Lebesgue outer measure is defined for any subset of (and is a -algebra).
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for any , and .
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If and are disjoint sets, then . More generally, if is a countable sequence of disjoint sets, then . This property is known as countable subadditivity and is weaker than countable additivity. In fact, is not countably additive.
Lebesgue outer measure has other nice properties:
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The outer measure of an interval is its length: .
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is translation-invariant. That is, if we define to be the set , we have for any .
The outer measure satisfies all the axioms of a measure except (countable) additivity. However, it is countably additive when one restricts to at least the Borel sets, as this is the usual construction of Borel measure. This result is roughly contained in the Caratheodory Extension theorem.
Title | Lebesgue outer measure |
Canonical name | LebesgueOuterMeasure |
Date of creation | 2013-03-22 11:48:15 |
Last modified on | 2013-03-22 11:48:15 |
Owner | yark (2760) |
Last modified by | yark (2760) |
Numerical id | 14 |
Author | yark (2760) |
Entry type | Definition |
Classification | msc 28A12 |
Synonym | outer measure |
Related topic | Infimum |
Related topic | LebesgueMeasure |
Related topic | ProofThatTheOuterLebesgueMeasureOfAnIntervalIsItsLength |
Related topic | CaratheodorysLemma |
Related topic | ConstructionOfOuterMeasures |