Topological group

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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

Caution about algebraic groups

Further information: Algebraic groups are not topological groups

Algebraic groups can be given a topology arising from their algebraic variety structure, namely, the Zariski topology. However, with some trivial exceptions, algebraic groups are not topological groups. The reason is that the multiplication map is not jointly continuous. Another way of seeing this is that algebraic groups are T_1 but not Hausdorff under the Zariski topology, but we know that for topological groups, being T_1 is equivalent to being Hausdorff.

However, there are two saving graces:

  • Algebraic groups are quasitopological groups, where a quasitopological group is a group with a topology where the inverse map is continuous and the group multiplication map is separately continuous in each variable.
  • Algebraic groups over a topological field are topological groups with respect to the topology arising from the field topology, and in the case that the field comes equipped with analytic structure, they become Lie groups over the field. For instance, real algebraic groups are real Lie groups and complex algebraic groups are complex Lie groups.

Relation with other structures

Stronger structures

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
H-group topological space with binary operation, constant element, and inverse map, all continuous, satisfying the group axioms up to homotopy |FULL LIST, MORE INFO

References

Textbook references

  • Topology (2nd edition) by James R. MunkresMore info, Page 145, Supplementary Exercises (assumes T_0 in the definition of topological group)