# Difference between revisions of "Open set"

From Encyclopedia of Mathematics

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''in a topological space'' | ''in a topological space'' | ||

− | An element of the topology (cf. [[ | + | An element of the topology (cf. [[Topological structure (topology)]]) of this space. More specifically, let the topology $\tau$ of a topological space $(X, \tau)$ be defined as a system $\tau$ of subsets of the set $X$ such that: |

# $X\in\tau$, $\emptyset\in\tau$; | # $X\in\tau$, $\emptyset\in\tau$; | ||

# if $O_i\in\tau$, where $i=1,2$, then $O_1\cap O_2\in\tau$; | # if $O_i\in\tau$, where $i=1,2$, then $O_1\cap O_2\in\tau$; | ||

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====References==== | ====References==== | ||

<table><TR><TD valign="top">[a1]</TD> <TD valign="top"> R. Engelking, "General topology" , Heldermann (1989)</TD></TR></table> | <table><TR><TD valign="top">[a1]</TD> <TD valign="top"> R. Engelking, "General topology" , Heldermann (1989)</TD></TR></table> | ||

+ | |||

+ | [[Category:General topology]] |

## Revision as of 17:21, 14 October 2014

*in a topological space*

An element of the topology (cf. Topological structure (topology)) of this space. More specifically, let the topology $\tau$ of a topological space $(X, \tau)$ be defined as a system $\tau$ of subsets of the set $X$ such that:

- $X\in\tau$, $\emptyset\in\tau$;
- if $O_i\in\tau$, where $i=1,2$, then $O_1\cap O_2\in\tau$;
- if $O_{\alpha}\in\tau$, where $\alpha\in\mathfrak{A}$, then $\bigcup\{O_{\alpha} : \alpha\in\mathfrak{A} \}$.

The open sets in the space $(X, \tau)$ are then the elements of the topology $\tau$ and only them.

#### References

[a1] | R. Engelking, "General topology" , Heldermann (1989) |

**How to Cite This Entry:**

Open set.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Open_set&oldid=29690

This article was adapted from an original article by B.A. Pasynkov (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article