This is a problem 1 from Munkres pg 83. I'm trying to solve for self study.(adsbygoogle = window.adsbygoogle || []).push({});

Let X be a topological space; let A be a subset of X. Suppose that for each x belonging in A there is an open set U containing x such that U is a subset of A. Show that A is open in X.

I'm not sure exactly how an open subset of A makes the whole A open.

Thanks

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# Topology question?

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