*Definition [#dceab476]
-A topological space (X,τ) is said to be pre-D_0 if for any distinct pair of points x and y of X , there exists a [[pD-set>pre-Difference set]] of X containing x but not y or a [[pD-set>pre-Difference set]] of X containing y but not x.

*Reference [#wad4445f]
-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.