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Let $A$ be an infinite set and $B$ be a countable set. I want to show that $|A|=|A\cup B|$.

I'm aware that the following relevant definitions:

  • An infinite set is one which has a proper subset of equal cardinality.
  • A countable set is one which is finite or countably infinite.
  • We have $|A|=|A\cup B|$, for sets $A$ and $B$, precisely when there exists a bijection $A\to A\cup B$.

Furthermore, it seems like it will be easier to use the Cantor-Berstein theorem rather than explicitly constructing a bijection. However I'm not sure if using surjective maps is more effective than injective maps (vice versa) for solving this problem. Another important fact is that if $A$ is infinite and $|A|=|A'|$ for some set $A'$, then $A'$ is also infinite.

Asaf Karagila
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    I am pretty sure this has uncountably many duplicates by now. – Asaf Karagila Jun 18 '16 at 06:53
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    For example, http://math.stackexchange.com/questions/355049/let-a-be-any-uncountable-set-and-let-b-be-a-countable-subset-of-a-prove is the same question, essentially, dressed in a different wording. – Asaf Karagila Jun 18 '16 at 06:57

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