# Uniform continuity

A property of a function (mapping) $ f: X \rightarrow Y $,
where $ X $
and $ Y $
are metric spaces. It requires that for any $ \epsilon > 0 $
there is a $ \delta > 0 $
such that for all $ x _ {1} , x _ {2} \in X $
satisfying $ \rho ( x _ {1} , x _ {2} ) < \delta $,
the inequality $ \rho ( f ( x _ {1} ), f ( x _ {2} )) < \epsilon $
holds.

If a mapping $ f: X \rightarrow Y $ is continuous on $ X $ and $ X $ is a compactum, then $ f $ is uniformly continuous on $ X $. The composite of uniformly-continuous mappings is uniformly continuous.

Uniform continuity of mappings occurs also in the theory of topological groups. For example, a mapping $ f: X _ {0} \rightarrow Y $, where $ X _ {0} \subset X $, $ X $ and $ Y $ topological groups, is said to be uniformly continuous if for any neighbourhood of the identity $ U _ {y} $ in $ Y $, there is a neighbourhood of the identity $ U _ {x} $ in $ X $ such that for any $ x _ {1} , x _ {2} \in X _ {0} $ satisfying $ x _ {1} x _ {2} ^ {-} 1 \in U _ {x} $( respectively, $ x _ {1} ^ {-} 1 x _ {2} \in U _ {x} $), the inclusion $ f ( x _ {1} ) [ f ( x _ {2} )] ^ {-} 1 \in U _ {y} $( respectively, $ [ f ( x _ {1} )] ^ {-} 1 f ( x _ {2} ) \in U _ {y} $) holds.

The notion of uniform continuity has been generalized to mappings of uniform spaces (cf. Uniform space).

#### References

[1] | A.N. Kolmogorov, S.V. Fomin, "Elements of the theory of functions and functional analysis" , 1–2 , Graylock (1957–1961) (Translated from Russian) |

[2] | L.S. Pontryagin, "Topological groups" , Princeton Univ. Press (1958) (Translated from Russian) |

[3] | J.L. Kelley, "General topology" , Springer (1975) |

[4] | N. Bourbaki, "General topology" , Elements of mathematics , Springer (1989) (Translated from French) |

#### Comments

There are several natural uniform structures on a topological group; the (confusing) statement above about uniform continuity of mappings between them can be interpreted in various ways.

#### References

[a1] | W. Roelcke, S. Dierolf, "Uniform structures on topological groups and their quotients" , McGraw-Hill (1981) |

[a2] | R. Engelking, "General topology" , Heldermann (1989) |

**How to Cite This Entry:**

Uniform continuity.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Uniform_continuity&oldid=49070