Talk:Characterization of Closure by Open Sets

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)