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If $A$ is an infinite set and $B$ is denumerable, $A$ is equipotent with the union $A\cup B$.

How to prove this?

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    I havent learnt what is 'cardinal number' yet...2012-05-07

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Let $B'=B\setminus A$. If $B'$ is finite, then biject $B$ with $A\cap B$, and use that bijection to biject $A\cup B = A\amalg B'$ with $A$ (where $\amalg$ denotes a disjoint union).

If $B'$ is infinite, let $D$ be a denumerable subset of $A$. Then biject $D\amalg B'$ with $D$, and use that bijection to get a bijection of $A\cup B = A\amalg B'$ with $A$.

(This requires you to prove that the union of two disjoint denumerable sets is denumerable; I trust you know how to do that)

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    @Katlus: I suspect there are ways of doing it, but I can't think of any off the top of my head; you could break up the cases with $B'$ infinite according to whether $B\cap A$ is finite or infinite, but I don't think it will be any simpler2012-05-08