# proof that a relation is union of functions if and only if AC

###### Theorem.

A relation  $R$ is the union of a set of functions  each of which has the same domain as $R$ if and only if for each set $A$ of nonempty sets, there is a choice function on $A$.

###### Proof.

Suppose that $R$ is a relation with $\operatorname{dom}(R)=A\mbox{ and }\operatorname{rng}(R)\subseteq B$. Let $g\colon A\to\mathcal{P}(B)$ be given by $a\mapsto R\left[\{a\}\right]$. There is be a choice function $c$ on $g\left[A\right]$. Let $f=c\circ g$, and for each pair $\langle a,b\rangle\in R$, let $f_{ab}$ send $a$ to $b$ and agree with $f$ elsewhere. Let $F=\{f_{ab}\mid\langle a,b\rangle\in R\}$. Clearly $\bigcup F\subseteq A\times B$, so suppose $\langle u,v\rangle\in\bigcup F$; then there is a pair $\langle a,b\rangle\in R$ such that $\langle u,v\rangle\in f_{ab}\in F$. Either $\langle u,v\rangle=\langle a,b\rangle$, or $v=f(u)=c\circ g(u)=c(R\left[\{u\}\right])\in R\left[\{u\}\right]$. In each case, $\langle u,v\rangle\in R$. Thus, $\bigcup F\subseteq R.$ For each pair $\langle a,b\rangle\in R$, $\langle a,b\rangle\in f_{ab}\in F$, so $R\subseteq\bigcup F$. Therefore, $R=\bigcup F$.
Suppose that $A$ is set of nonempty sets. Let $R=\bigcup\{\{a\}\times a\mid a\in A\}$. A set $x$ is an element of $\operatorname{dom}(R)$ if and only if $x\in\{a\}$ for some $a\in A$. Thus, $\operatorname{dom}(R)=A$. There is a set $F$ of functions, each of which has domain $A$, such that $R=\bigcup F$. Let $f\in F$; then $\operatorname{dom}(f)=A$, and for each pair $\langle a,f(a)\rangle\in f$, $\langle a,f(a)\rangle\in\{a\}\times a$; i.e., $f(a)\in a$. Each such $f$ is, thus, a choice function on $A$. ∎

Title proof that a relation is union of functions if and only if AC ProofThatARelationIsUnionOfFunctionsIfAndOnlyIfAC 2013-03-22 16:34:23 2013-03-22 16:34:23 ratboy (4018) ratboy (4018) 4 ratboy (4018) Proof msc 03E25