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countably infinite union of countably infinite sets is countable
proof that union of a sequence of countable sets is countable.

I'm a newbie who try to understand Set Theory. Is there anybody who can explain the solution for the following problem?

Assume that C is a countable set of countable pairwise disjoint sets, how can we prove that $\cup C$ is countable?

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    @Asaf karagila Yes, I realized that later, so I deleted my comment accordingly.2012-05-14

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