Point-set topology is a fundamental branch of topology, sometimes referred to as general topology, which deals with the concepts of topological spaces and the mathematical structures defined on such spaces.

Topology and open sets

Given a set , a family of subsets of is said to be a topology of if the following three conditions hold:

  1. (The empty set and are both elements of )
  2. (Any union of elements of is an element )
  3. (Any finite intersection of elements of is an element of )

The members of a topology are called open sets of the topology.

Topological space

A topological space is a set , known as the underlying set, together with a topology T of .

Basis for a topology

A basis for a topology on is a collection of subsets of , known as basis elements, such that the following two properties hold:

  1. For every there is at least one basis element that contains .
  2. If is an element of the intersection of two basis elements , then there exists a basis element such that .

Given a basis for a topology, one can define the topology generated by the basis as the collection of all sets such that for each there is a basis element such that and .

Closed sets

A set is defined to be closed if its complement in is an open set in the given topology.


A set is said to be a neighborhood of a point if it is an open set which contains the point . In some cases the term neighborhood is used to describe a set which contains an open set containing .

Interior and closure

The interior of a subset of is defined to be the union of all open sets contained in .

The closure of a subset of is defined as the intersection of all closed sets containing .

Limit points

A point of is said to be a limit point of a subset A of if every neighborhood of intersects A in at least one point other than .

Continuous functions

A function is said to be continuous if for each subset of , the set is an open set of .


A bijective function is said to be a homeomorphism if both and its inverse, , are continuous.

If there exists a homeomorphism between two topological spaces X and Y, then the spaces are said to be homeomorphic.

Any property that is invariant under homeomorphisms is known as a topological property.

A homeomorphism is also dubbed a topological equivalence among mathematicians.

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