*Definition [#a8d39aa3]
-Th complement of [[η-open]] sets are called η-closed.

*Reference [#cc2fbcab]
-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.