*Definition [#k671b842]
-Let (X,τ) be a topological space. A subset S of X is called a Λ-set if it is equal to the intersection of all open supersets of S.

*Reference [#a25d5bd6]
-Caldas Miguel, Jafari Saeid, Navalagi Govindappa, ''More on λ-closed sets in topological spaces.'',Rev. Colombiana Mat. 41 (2007), no. 2, 355-369.