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| :''This page gives a general overview of the concept of non-measurable sets. For a precise definition of measure, see [[Measure (mathematics)]]. For various constructions of non-measurable sets, see [[Vitali set]], [[Hausdorff paradox]], and [[Banach–Tarski paradox]]''.
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| In [[mathematics]], a '''non-measurable set''' is a [[Set (mathematics)|set]] which cannot be assigned a meaningful "size". The [[mathematical existence]] of such sets is construed to shed light on the notions of [[length]], [[area]] and [[volume]] in formal set theory.
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| The notion of a non-measurable set has been a source of great controversy since its introduction. Historically, this led [[Émile Borel|Borel]] and [[Kolmogorov]] to formulate probability theory on sets which are constrained to be measurable. The measurable sets on the line are iterated countable unions and intersections of intervals (called [[Borel set]]s) plus-minus [[null set]]s. These sets are rich enough to include every conceivable definition of a set that arises in standard mathematics, but they require a lot of formalism to prove that sets are measurable.
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| In 1970, [[Robert M. Solovay|Solovay]] constructed [[Solovay's model]], which shows that it is consistent with standard set theory, excluding uncountable choice, that all subsets of the reals are measurable.
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| == Historical constructions ==
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| The first indication that there might be a problem in defining length for an arbitrary set came from [[Vitali set|Vitali's theorem]].{{Citation needed|date=June 2008}}
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| When you form the union of two disjoint sets, one would expect the measure of the result to be the sum of the measure of the two sets. A measure with this natural property is called ''finitely additive''. While a finitely additive measure is sufficient for most intuition of area, and is analogous to [[Riemann integration]], it is considered insufficient for [[probability]], because conventional modern treatments of sequences of events or random variables demand [[countable additivity]].
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| In this respect, the plane is similar to the line; there is a finitely additive measure, extending Lebesgue measure, which is invariant under all [[isometries]]. When you increase in [[dimension]] the picture gets worse. The [[Hausdorff paradox]] and [[Banach–Tarski paradox]] show that you can take a three dimensional [[ball (mathematics)|ball]] of radius 1, dissect it into 5 parts, move and rotate the parts and get two balls of radius 1. Obviously this construction has no meaning in the physical world. In 1989, [[A. K. Dewdney]] published a letter from his friend Arlo Lipof in the Computer Recreations column of the ''[[Scientific American]]'' where he describes an underground operation "in a South American country" of doubling gold balls using the [[Banach–Tarski paradox]].<ref>Dewdney (1989)</ref> Naturally, this was in the April issue, and "Arlo Lipof" is an [[anagram]] of "[[April Fools' Day|April Fool]]".
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| ==Example==
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| Consider the unit circle ''S'', and the [[Group_action|action]] on ''S'' by a group ''G'' consisting of all rational rotations. Namely, these are rotations by angles which are rational multiples of π. Here ''G'' is countable (more specifically, ''G'' is isomorphic to <math>\mathbb{Q}/\mathbb{Z}</math>) while ''S'' is uncountable. Hence ''S'' breaks up into uncountably many orbits under ''G''. Using the [[axiom of choice]], we could pick a single point from each orbit, obtaining an uncountable subset <math>X \subset S</math> with the property that all of its translates by ''G'' are disjoint from ''X'' and from each other. The set of those translates partitions the circle into a countable collection of disjoint sets, which are all pairwise congruent (by rational rotations). The set ''X'' will be non-measurable for any rotation-invariant countably additive probability measure on ''S'': if ''X'' has zero measure, countable additivity would imply that the whole circle has zero measure. If ''X'' has positive measure, countable additivity would show that the circle has infinite measure.
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| == Consistent definitions of measure and probability ==
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| The [[Banach–Tarski paradox]] shows that there is no way to define volume in three dimensions unless one of the following four concessions is made:
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| # The volume of a set might change when it is rotated
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| # The volume of the union of two disjoint sets might be different from the sum of their volumes
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| # Some sets might be tagged "non-measurable" and one would need to check if a set is "measurable" before talking about its volume
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| # The axioms of ZFC ([[Zermelo–Fraenkel set theory]] with the axiom of Choice) might have to be altered
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| Standard measure theory takes the third option. One defines a family of measurable sets which is very rich, and almost any set explicitly defined in most branches of mathematics will be among this family. It is usually very easy to prove that a given specific subset of the geometric plane is measurable. The fundamental assumption is that a countably infinite sequence of disjoint sets satisfies the sum formula, a property called [[sigma additivity|σ-additivity]].
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| In 1970, [[Robert M. Solovay|Solovay]] demonstrated that the existence of a non-measurable set for the [[Lebesgue measure]] is not provable within the framework of Zermelo–Fraenkel set theory in the absence of the Axiom of Choice, by showing that (assuming the consistency of an [[inaccessible cardinal]]) there is a model of ZF, called [[Solovay's model]], in which [[countable choice]] holds, every set is Lebesgue measurable and in which the full axiom of choice fails.
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| The Axiom of Choice is equivalent to a fundamental result of [[point-set topology]], [[Tychonoff's theorem]], and also to the conjunction of two fundamental results of functional analysis, the [[Banach–Alaoglu theorem]] and the [[Krein–Milman theorem]]. It also affects the study of infinite groups to a large extent, as well as ring and order theory (see [[Boolean prime ideal theorem]]). However the axioms of [[determinacy]] and [[dependent choice]], together, are sufficient for most [[geometric measure theory]], [[potential theory]], [[Fourier series]] and [[Fourier transforms]], while making all subsets of the real line Lebesgue measurable.
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| == See also ==
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| * [[Non-Borel set]]
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| ==References==
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| ===Notes===
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| {{reflist}}
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| ===Bibliography===
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| {{refbegin}}
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| *{{cite journal|last=Dewdney|first=A. K.|date=1989|title=A matter fabricator provides matter for thought|journal=Scientific American|issue=April|pages=116–119}}
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| {{refend}}
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| {{DEFAULTSORT:Non-Measurable Set}}
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| [[Category:Measure theory]]
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