Ok, the proof looks simple since by defintion Cl A = intersection of all closed sets containing A. And textbooks give a quick proof that we all understand, but I have a question: Don't we first have to prove that a smallest closed set containing A exists in the first place?(adsbygoogle = window.adsbygoogle || []).push({});

I'm trying to prove the existence using Zorn's Lemma...

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# Closure of A is the smallest closed set containing A

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