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A set (in light blue) and its boundary (in dark blue). In topology and mathematics in general, the boundary of a subset S of a topological space X is the set of points in the closure of S not belonging to the interior of S. An element of the boundary of S is called a boundary point of S.
The point x is an interior point of S. The point y is on the boundary of S. In mathematics, specifically in topology, the interior of a subset S of a topological space X is the union of all subsets of S that are open in X. A point that is in the interior of S is an interior point of S. The interior of S is the complement of the closure of the ...
Each clopen subset of (which includes and itself) is simultaneously a regular open subset and regular closed subset. The interior of a closed subset of is a regular open subset of and likewise, the closure of an open subset of is a regular closed subset of . [2] The intersection (but not necessarily the union) of two regular open sets is a ...
A subset of a space X is regular open if it equals the interior of its closure; dually, a regular closed set is equal to the closure of its interior. [21] An example of a non-regular open set is the set U = (0,1) ∪ (1,2) in R with its normal topology, since 1 is in the interior of the closure of U, but not in U.
For another example, consider the relative interior of a closed disk in . It is locally closed since it is an intersection of the closed disk and an open ball. On the other hand, { ( x , y ) ∈ R 2 ∣ x ≠ 0 } ∪ { ( 0 , 0 ) } {\displaystyle \{(x,y)\in \mathbb {R} ^{2}\mid x\neq 0\}\cup \{(0,0)\}} is not a locally closed subset of R 2 ...
Theorem — If is nonempty and convex, then its relative interior () is the union of a nested sequence of nonempty compact convex subsets (). Proof Since we can always go down to the affine span of A {\displaystyle A} , WLOG, the relative interior has dimension n {\displaystyle n} .
The interior of every set except X is empty. The closure of every non-empty subset of X is X. Put another way: every non-empty subset of X is dense, a property that characterizes trivial topological spaces. As a result of this, the closure of every open subset U of X is either ∅ (if U = ∅) or X (otherwise).
Assume that is a subset of a vector space . The algebraic interior (or radial kernel) of with respect to is the set of all points at which is a radial set.A point is called an internal point of [1] [2] and is said to be radial at if for every there exists a real number > such that for every [,], +.