# Definition:Path-Connected

## Definition

## Topology

### Points in Topological Space

Let $T = \struct {S, \tau}$ be a topological space.

Let $a, b \in S$ be such that there exists a path from $a$ to $b$.

That is, there exists a continuous mapping $f: \closedint 0 1 \to S$ such that:

- $\map f 0 = a$

and:

- $\map f 1 = b$

Then $a$ and $b$ are **path-connected in $T$**.

### Set of Topological Space

Let $T = \struct {S, \tau}$ be a topological space.

Let $U \subseteq S$ be a subset of $S$.

Let $T' = \struct {U, \tau_U}$ be the subspace of $T$ induced by $U$.

Then $U$ is a **path-connected set in $T$** if and only if every two points in $U$ are path-connected in $T\,'$.

That is, $U$ is a **path-connected set in $T$** if and only if:

- for every $x, y \in U$, there exists a continuous mapping $f: \closedint 0 1 \to U$ such that:
- $\map f 0 = x$

- and:
- $\map f 1 = y$

### Topological Space

Let $T = \struct {S, \tau}$ be a topological space.

Then $T$ is a **path-connected space** if and only if $S$ is a path-connected set of $T$.

That is, $T$ is a **path-connected space** if and only if:

- for every $x, y \in S$, there exists a continuous mapping $f: \closedint 0 1 \to S$ such that:
- $\map f 0 = x$

- and:
- $\map f 1 = y$

## Metric Space

Let $M = \struct {A, d}$ be a metric space.

$M$ is defined as **path-connected** if and only if:

- $\forall m, n \in A: \exists f: \closedint 0 1 \to A: \map f 0 = m, \map f 1 = n$

where $f$ is a continuous mapping.

### Subset of Metric Space

Let $M = \struct {A, d}$ be a metric space.

Let $S \subseteq A$ be a subset of $M$.

Then $S$ is **path-connected (in $M$)** if and only if:

- $\forall m, n \in S: \exists f: \closedint 0 1 \to S: \map f 0 = m, \map f 1 = n$

where $f$ is a continuous mapping.

## Also known as

Some sources do not hyphenate **path-connected**, but instead report this as **path connected**.

Some sources use the term **arc-connected**, but this normally has a **more precise meaning**.

## Also see

- Results about
**path-connected spaces**can be found**here**.

## Sources

- 1989: Ephraim J. Borowski and Jonathan M. Borwein:
*Dictionary of Mathematics*... (previous) ... (next):**arc-connected**:**1.**