*Definition [#b77559aa]
-Let (X,τ) be a topological space. A subset S of X is said to be α-set if S ⊂ Int(cl(Int(S))).

*Reference [#bb03fa0e]
-El-Deeb, N.; Hasanein, I. A.; Mashhour, A. S.; Noiri, T. , ''On p-regular spaces.'', Bull. Math. Soc. Sci. Math. R. S. Roumanie (N.S.) 27(75) (1983), no. 4, 311–315.