*Definition [#e0367c3b]
-The derived set of a subset S of a topological space is the set of all limit points of S.

*Reference [#w683eab1]
-Kelley, John L.,''General topology. 2nd ed.'' (English) [B] Graduate Texts in Mathematics. 27. New York - Heidelberg - Berlin: Springer-Verlag. XIV, 298 p