Definition
- A topological space (X,τ) is said to be T_{DD} if X is T_D and for all (where {x}' is the derived set of {x}).
Property
- T_{DD} ⇒ T_YS?
Reference
- Aull, Charles E.; Thron, W.J.,Separation axioms between T0 and T1., (English) [J] Nederl. Akad. Wet., Proc., Ser. A 65, 26-37 (1962).