Topological group

This article gives a basic definition in the following area: topological group theory
View other basic definitions in topological group theory |View terms related to topological group theory |View facts related to topological group theory

This article describes a compatible combination of two structures: group and topological space

This article defines the notion of group object in the category of topological spaces|View other types of group objects

This is a variation of group|Find other variations of group | Read a survey article on varying group

Definition

Abstract definition

The notion of topological group can be defined in the following equivalent ways:

1. In the language of universal algebra, it is a group equipped with a topology for which all the defining operations of groups are continuous.
2. It is a group object in the category of topological spaces.

Concrete definition

A topological group is a set $G$ endowed with the following two structures:

• The structure of a group, i.e., a binary operation called multiplication or product, a constant called the identity element, and a unary operation called the inverse map, and satisfying the conditions for a group
• The structure of a topological space

such that the following compatibility conditions are satisfied:

Operation	Arity	Condition	Comments
Multiplication or product	2 (so it's a map $G \times G \to G$)	continuous as a map from $G \times G$ (equipped with the product topology) to $G$. In other words, the group multiplication $(g,h) \mapsto gh$ is jointly continuous.	Joint continuity is strictly stronger than separate continuity, which would mean continuity in each input holding the other input fixed.
Identity element	0 (it's a constant element $e \in G$)	No condition. As such, we may impose the condition that the map from a one-point space to $G$ sending the point to the identity element is continuous, but this condition is vacuously true.
Inverse map	1 (so it's a map $G \to G$)	continuous as a map from $G$ to itself with the equipped topology. In other words, $g \mapsto g^{-1}$ is continuous.	Note that because the inverse map is its own inverse (see inverse map is involutive), this is equivalent to it being a self-homeomorphism of $G$.

T0 assumption

Some people assume a topological group to be $T_0$, that is, that there is no pair of points with each in the closure of the other. This is not a very restrictive assumption, because if we quotient out a topological group by the closure of the identity element, we do get a $T_0$-topological group. However, the definition above does not include this assumption. Further information: T0 topological group

Relation with other structures

Stronger structures

• Lie group

Weaker structures

Structure	Meaning	Proof of implication	Proof of strictness (reverse implication failure)	Intermediate notions
paratopological group	multiplication is still required to be jointly continuous, but there is no condition on the inverse map.
quasitopological group	multiplication is required to only be separately continuous (which is weaker than jointly continuous), and the inverse map is required to be continuous.
semitopological group	multiplication is required to only be separately continuous (which is weaker than jointly continuous), and there is no condition on the inverse map.			Paratopological group, Quasitopological group|FULL LIST, MORE INFO

References

Textbook references

• Topology (2nd edition) by James R. Munkres^{More info}, Page 145, Supplementary Exercises (assumes $T_0$ in the definition of topological group)