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Example: the blue circle represents the set of points (x, y) satisfying x 2 + y 2 = r 2.The red disk represents the set of points (x, y) satisfying x 2 + y 2 < r 2.The red set is an open set, the blue set is its boundary set, and the union of the red and blue sets is a closed set.
A subset of is a regular open set if and only if its complement in is a regular closed set. [2] Every regular open set is an open set and every regular closed set is a closed set. Each clopen subset of (which includes and itself) is simultaneously a regular open subset and regular closed subset.
Regular open set, regular closed set; Connected set; Perfect set; Meagre set; Nowhere dense set; Relative to a metric. Bounded set; Totally bounded set;
For example, the Euclidean topology on the plane admits as a base the set of all open rectangles with horizontal and vertical sides, and a nonempty intersection of two such basic open sets is also a basic open set. But another base for the same topology is the collection of all open disks; and here the full (B2) condition is necessary.
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. The regular open subsets of a space form a complete Boolean algebra .
Any set can be given the cofinite topology in which the open sets are the empty set and the sets whose complement is finite. This is the smallest T 1 topology on any infinite set. Any set can be given the cocountable topology, in which a set is defined as open if it is either empty or its complement is countable. When the set is uncountable ...
If the space X is a metric space, for example a Euclidean space, then an element x of S is an isolated point of S if there exists an open ball around x that contains only finitely many elements of S. A point set that is made up only of isolated points is called a discrete set or discrete point set (see also discrete space).
Since any set is open, the complement of any set is open too, and therefore any set is closed. So, all sets in this metric space are clopen. As a less trivial example, consider the space Q {\displaystyle \mathbb {Q} } of all rational numbers with their ordinary topology, and the set A {\displaystyle A} of all positive rational numbers whose ...