Let $X\subset \mathbb{R}$.I have to prove,for all $X$ that $X´$,i.e, the set of accumulation points, is a closed set.
Well,I know , by definition, that every accumulation point is a point of closure.
How a set is said to be closed if $X = \overline X$ ,and how the accumulations points are
points of closure,then:
$X´=\overline X$.
So $X´ $ is closed.
Is that right?