*Definition [#uc5198e1]
-The complement of a [[β-open]] set is called a β-closed set.

*Reference [#fc33cff8]
-Basu, C. K.; Afsan, B. M. Uzzal; Ghosh, M. K., ''A class of functions and separation axioms with respect to an operation.'' (English summary), Hacet. J. Math. Stat. 38 (2009), no. 2, 103-118.