Preopen subset

Definition
A subset of a topological space is termed a preopen subset if it satisfies the following equivalent conditions:


 * 1) It is contained in the defining ingredient::interior of its defining ingredient::closure.
 * 2) It is the intersection of a defining ingredient::regular open subset and a defining ingredient::dense subset.
 * 3) It is the intersection of an defining ingredient::open subset and a dense subset.