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I read a question stating that if $z$ is complex, then $|z|\leq 1$ is a closed set. I think this is just saying that the unit disk is a closed set. Why is that so?

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    Follows from the defn of closed. For instance, the complement is open (why?)2011-08-13
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    The question http://math.stackexchange.com/questions/30039/a-subset-g-of-rn-is-open-iff-the-complement-of-g-is-closed might be helpful for reviewing properties of closed and open sets.2011-08-13

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