Suppose I have some subset of R, not necessarily an interval, let it be denoted as A. I have some union (might be countable, might be finite, might be uncountable) of sets where each set is an open set of A and the union of the open sets is equal to A. Can I conclude that A is open?(adsbygoogle = window.adsbygoogle || []).push({});

I am not sure because the sets are open in A, not necessarily open in R. I don't know much about A other than it is some subset of R.

Any help?

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# Homework Help: Topology question

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