regulaarsetel
Regulaarsetel is a term used in Estonian topology to denote a class of subsets of a topological space that satisfy a regularity condition with respect to the interior and closure operations. In practice, there are two closely related notions commonly described under this umbrella: regular open sets and regular closed sets.
Regular open sets: a set U is regular open if U = int(cl(U)). Regular closed sets: a set
Examples help illustrate the idea. In the real line with the standard topology, any open interval (a,b)
Usage and context: The term regulaarsetel appears mainly in Estonian-language texts on topology and geometry; in