*Definition [#h28ed510]
-A topological space (X,τ) is called pre-D_2 if for any distinct pair of points x and y of X , there exists disjoint [[pD-sets>pre-Difference set]] &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b%5c%5b%20U%5cmbox%7b%20and%20%7dV%5cmbox%7b%20such%20that%20%7d%20x%5cin%20U%5cmbox%7b%20and%20%7d%20y%5cin%20V%20%5c%5d%7d%25.png);.

*Property [#e279378d]
-If (X,τ) is pre-D_2, then it is [[pre-D_1]].

*Reference [#r20450c2]
-Caldas, M.; Georgiou, D. N.; Jafari, S. , ''Characterizations of low separation axioms via α-open sets and α-closure operator.'', (English summary) Bol. Soc. Parana. Mat. (3) 21 (2003), no. 1-2, 97–111.