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Possible Duplicate:
Using a choice function to find an inverse for $F\colon A\to P(B)$

Let $F:A \rightarrow \mathcal P (B)$ be arbitary functions which covers $B$.

Use AC to show there is a function $\phi: B \rightarrow A$ such that $b \in F( \phi(b))$ for each $b \in B$ .

Do I use AC on the set B. I don't see what I'm choosing here.

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    I think I answered this recently. On the iPhone, it's simpler to answer than to find duplicates... :-/2012-12-05

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For every $b\in B$ there is some $a\in A$ such that $b\in F(a)$. You need to choose such $a$ for every $b$.

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    @simplicity: Yes, that’s exactly right.2012-12-05