η-open

Definition

• A subset S of a topological space (X,τ) is called η-open if S is the union of a δ-closed sets.

Property

• Every η-open set is ∨-set?.

Reference

• Dontchev, J.; Popvassilev, S.; Stavrova, D., On the η-expansion topology for the co-semi-regularization and mildly Hausdorff spaces., (English summary), Acta Math. Hungar. 80 (1998), no. 1-2, 9-19.