*Definition [#kff1a2b2]
-A subset A of a topological space (X,τ) is called δ-open if &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b%5c%5b%20A=%5crm%7bInt%7d_%7b%5cdelta%7d%28A%29%20%5c%5d%7d%25.png);, where &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b%5c%5b%20%5crm%7bInt%7d_%7b%5cdelta%7d%28A%29%20%5c%5d%7d%25.png); is the [[δ-interior]] of the set A.
*Reference [#he7031b5]
-Caldas, M.; Ganster, M.; Georgiou, D. N.; Jafari, S.; Moshokoa, S. P., ''δ-semiopen sets in topology.'' (English summary), Proceedings of the 19th Summer Conference on Topology and its Applications., Topology Proc. 29 (2005), no. 2, 369–383.