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{{Short description|Mathematical function}}
{{for|the combinatorial choice function C(n, k)|Combination|Binomial coefficient}}

Let ''X'' be a [[set of sets]] none of which are empty. Then a '''choice function''' ('''selector''', '''selection''') on ''X'' is a [[mathematical function]] ''f'' that is defined on ''X'' such that ''f'' is a mapping that assigns each element of ''X'' to one of its elements.

== An example ==
Let ''X'' = { {1,4,7}, {9}, {2,7} }. Then the function ''f'' defined by ''f''({1, 4, 7}) = 7, ''f''({9}) = 9 and ''f''({2, 7}) = 2 is a choice function on ''X''.

== History and importance ==
[[Ernst Zermelo]] (1904) introduced choice functions as well as the [[axiom of choice]] (AC) and proved the [[well-ordering theorem]],<ref name="Zermelo, 1904">{{cite journal| first=Ernst| last=Zermelo| year=1904| title=Beweis, dass jede Menge wohlgeordnet werden kann| journal=Mathematische Annalen| volume=59| issue=4| pages=514–16| doi=10.1007/BF01445300| url=http://gdz.sub.uni-goettingen.de/no_cache/en/dms/load/img/?IDDOC=28526}}</ref> which states that every set can be [[well ordering|well-ordered]]. AC states that every set of nonempty sets has a choice function. A weaker form of AC, the [[axiom of countable choice]] (ACω) states that every [[countable set]] of nonempty sets has a choice function. However, in the absence of either AC or ACω, some sets can still be shown to have a choice function.

*If <math>X</math> is a [[finite set|finite]] set of nonempty sets, then one can construct a choice function for <math>X</math> by picking one element from each member of <math>X.</math> This requires only finitely many choices, so neither AC or ACω is needed.
*If every member of <math>X</math> is a nonempty set, and the [[union (set theory)|union]] <math>\bigcup X</math> is well-ordered, then one may choose the least element of each member of <math>X</math>. In this case, it was possible to simultaneously well-order every member of <math>X</math> by making just one choice of a well-order of the union, so neither AC nor ACω was needed. (This example shows that the well-ordering theorem implies AC. The [[Converse (logic)|converse]] is also true, but less trivial.)

== Choice function of a multivalued map ==
Given two sets <math>X</math> and <math>Y</math>, let <math>F</math> be a [[multivalued function|multivalued map]] from <math>X</math> to <math>Y</math> (equivalently, <math>F:X\rightarrow\mathcal{P}(Y)</math> is a function from <math>X</math> to the [[power set]] of <math>Y</math>).

A function <math>f: X \rightarrow Y</math> is said to be a '''selection''' of <math>F</math>, if:

<math display="block">\forall x \in X \, ( f(x) \in F(x) ) \,.</math>

The existence of more regular choice functions, namely continuous or measurable selections is important in the theory of [[differential inclusion]]s, [[optimal control]], and [[mathematical economics]].<ref>{{cite book
| last = Border
| first = Kim C.
| title = Fixed Point Theorems with Applications to Economics and Game Theory
| year = 1989
| publisher = Cambridge University Press
| isbn = 0-521-26564-9
}}</ref> See [[Selection theorem]].

==Bourbaki tau function==
[[Nicolas Bourbaki]] used [[epsilon calculus]] for their foundations that had a <math> \tau </math> symbol that could be interpreted as choosing an object (if one existed) that satisfies a given proposition. So if <math> P(x) </math> is a predicate, then <math>\tau_{x}(P)</math> is one particular object that satisfies <math>P</math> (if one exists, otherwise it returns an arbitrary object). Hence we may obtain quantifiers from the choice function, for example <math> P( \tau_{x}(P))</math> was equivalent to <math> (\exists x)(P(x))</math>.<ref>{{cite book|last=Bourbaki|first=Nicolas|title=Elements of Mathematics: Theory of Sets|date=1968 |publisher=Hermann |isbn=0-201-00634-0}}</ref>

However, Bourbaki's choice operator is stronger than usual: it's a ''global'' choice operator. That is, it implies the [[axiom of global choice]].<ref>John Harrison, "The Bourbaki View" [http://www.rbjones.com/rbjpub/logic/jrh0105.htm eprint].</ref> Hilbert realized this when introducing epsilon calculus.<ref>"Here, moreover, we come upon a very remarkable circumstance, namely, that all of these transfinite axioms are derivable from a single axiom, one that also contains the core of one of the most attacked axioms in the literature of mathematics, namely, the axiom of choice: <math>A(a)\to A(\varepsilon(A))</math>, where <math>\varepsilon</math> is the transfinite logical choice function." Hilbert (1925), “On the Infinite”, excerpted in Jean van Heijenoort, ''From Frege to Gödel'', p. 382. From [http://ncatlab.org/nlab/show/choice+operator nCatLab].</ref>

==See also==
* [[Axiom of countable choice]]
* [[Axiom of dependent choice]]
* [[Hausdorff paradox]]
* [[Hemicontinuity]]

==Notes==
{{Reflist}}

==References==
{{PlanetMath attribution|id=6419|title=Choice function}}

[[Category:Basic concepts in set theory]]
[[Category:Axiom of choice]]

Choice function - Revision history

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