
 PlanetMath: regular open set 
  It is not true, however, that the union of two regular open sets is regular open, as illustrated by the second example above. 
  Crossreferences: closed set, intersection, collection, operations, Boolean algebra, union, property, dense, open set, structure, open intervals, generated by, line, real, closure, interior, open, subset, topological space 
  This is version 2 of regular open set, born on 20050220, modified 20050222. 
 planetmath.org /encyclopedia/RegularlyClosed.html (208 words) 
