*Definition [#pf5c0369] A subset A of a topological space X is called regular open if &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b$A=%5cmathrm%7bint%7d%28%5cmathrm%7bcl%7d%28A%29%29$%7d%25.png);.
*Definition [#pf5c0369] A subset A of a topological space X is called regular open if &ref(http://www.eaflux.com/imgtex/imgtex.fcgi?%5bres=100%5d%7b$A=%5cmathrm%7bint%7d%28%5cmathrm%7bcl%7d%28A%29%29$%7d%25.png);.