*Definition [#c50ec663]
-Let S and X be sets. 
-The ablolute complement of a set S, written ～S, is &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b%5c%5b%20%5c%7bx%5c%20%7c%5c%20x%5cnotin%20S%5c%7d%20%5c%5d%7d%25.png);.
-The relative complement of S with respect to X is X ⋂ (～S), or simply X～S.

*Reference [#wb004998]
-Kelley, John L.,''General topology. 2nd ed.'' (English) [B] Graduate Texts in Mathematics. 27. New York - Heidelberg - Berlin: Springer-Verlag. XIV, 298 p (1975).