*Definition [#ic8d4798]
-A topological space (X,τ) is said to be T_{DD} if X is [[T_D]] and for all &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b%5c%5b%20x%2cy%5cin%20X%2c%5c%20x%5cneq%20y%2c%5c%20%5c%7bx%5c%7d'%5ccap%5c%7by%5c%7d'=%5cemptyset%20%5c%5d%7d%25.png); (where {x}' is the [[derived set]] of {x}). 

*Property [#s2bec1f3]
-T_{DD} ⇒　[[T_YS]]

*Reference [#sc9e7c68]
-Aull, Charles E.; Thron, W.J.,''Separation axioms between T0 and T1.'', (English) [J] Nederl. Akad. Wet., Proc., Ser. A 65, 26-37 (1962).