# Totally disconnected space

In topology and related branches of mathematics, a **totally disconnected space** is a topological space that is maximally disconnected, in the sense that it has no non-trivial connected subsets. In every topological space the empty set and the one-point sets are connected; in a totally disconnected space these are the *only* connected subsets.

An important example of a totally disconnected space is the Cantor set. Another example, playing a key role in algebraic number theory, is the field *Q*_{p} of *p*-adic numbers.

## Definition

A topological space *X* is **totally disconnected** if the connected components in *X* are the one-point sets. Analogously, a topological space *X* is **totally path-disconnected** if all path-components in *X* are the one-point sets.

## Examples

The following are examples of totally disconnected spaces:

- Discrete spaces
- The rational numbers
- The irrational numbers
- The p-adic numbers; more generally, profinite groups are totally disconnected.
- The Cantor set
- The Baire space
- The Sorgenfrey line
- Zero-dimensional T
_{1}spaces - Extremally disconnected Hausdorff spaces
- Stone spaces
- The Knaster–Kuratowski fan provides an example of a connected space, such that the removal of a single point produces a totally disconnected space.
- The Erdős space ℓ
^{p}(**Z**)∩ is a totally disconnected space that does not have dimension zero.

## Properties

- Subspaces, products, and coproducts of totally disconnected spaces are totally disconnected.
- Totally disconnected spaces are T
_{1}spaces, since singletons are closed. - Continuous images of totally disconnected spaces are not necessarily totally disconnected, in fact, every compact metric space is a continuous image of the Cantor set.
- A locally compact Hausdorff space is zero-dimensional if and only if it is totally disconnected.
- Every totally disconnected compact metric space is homeomorphic to a subset of a countable product of discrete spaces.
- It is in general not true that every open set is also closed.
- It is in general not true that the closure of every open set is open, i.e. not every totally disconnected Hausdorff space is extremally disconnected.

## Constructing a disconnected space

Let be an arbitrary topological space. Let if and only if (where denotes the largest connected subset containing ). This is obviously an equivalence relation. Endow with the quotient topology, i.e. the coarsest topology making the map continuous. With a little bit of effort we can see that is totally disconnected. We also have the following universal property: if a continuous map to a totally disconnected space, then it *uniquely* factors into where is continuous.

## References

- Willard, Stephen (2004),
*General topology*, Dover Publications, ISBN 978-0-486-43479-7, MR 2048350 (reprint of the 1970 original, MR 0264581)