Talk:Characterization of Closure by Open Sets
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Q:Why should the above hold?
Because, the $\complement_S(A^-)$ is open set and substitution of $U$ for $\complement_S(A^-)$ in the assumption gives the resukt.
- Yes of course. I have added that obvious statement. --prime mover (talk) 17:54, 24 September 2016 (EDT)