# Equivalence of Definitions of Component

## Theorem

The following definitions of the concept of **Component** in the context of **Topology** are equivalent:

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

Let $x \in T$.

### Definition 1: Equivalence Class

From Connectedness of Points is Equivalence Relation, $\sim$ is an equivalence relation.

From the Fundamental Theorem on Equivalence Relations, the points in $T$ can be partitioned into equivalence classes.

These equivalence classes are called the **(connected) components** of $T$.

If $x \in S$, then the **component of $T$ containing $x$** (that is, the set of points $y \in S$ with $x \sim y$) is denoted by $\map {\operatorname {Comp}_x} T$.

### Definition 2: Union of Connected Sets

The **component of $T$ containing $x$** is defined as:

- $\ds \map {\operatorname{Comp}_x} T = \bigcup \leftset{A \subseteq S: x \in A \land A}$ is connected $\rightset{}$

### Definition 3: Maximal Connected Set

The **component of $T$ containing $x$** is defined as:

- the maximal connected set of $T$ that contains $x$.

## Proof

Let $\CC_x = \set {A \subseteq S : x \in A \land A \text{ is connected in } T}$

Let $C = \bigcup \CC_x$

### Lemma

- $C$ is connected in $T$ and $C \in \CC_x$.

$\Box$

Let $C'$ be the equivalence class containing $x$ of the equivalence relation $\sim$ defined by:

- $y \sim z$ if and only if $y$ and $z$ are connected in $T$.

### Equivalence Class equals Union of Connected Sets

It needs to be shown that $C = C'$.

\(\ds y \in C'\) | \(\leadstoandfrom\) | \(\ds \exists B \text{ a connected set of } T, x \in B, y \in B\) | Definition of $\sim$ | |||||||||||

\(\ds \) | \(\leadstoandfrom\) | \(\ds \exists B \in \CC_x : y \in B\) | equivalent definition | |||||||||||

\(\ds \) | \(\leadstoandfrom\) | \(\ds y \in \bigcup \CC_x\) | Definition of Union of Set of Sets | |||||||||||

\(\ds \) | \(\leadstoandfrom\) | \(\ds y \in C\) | Definition of $C$ |

The result follows.

$\Box$

### Union of Connected Sets is Maximal Connected Set

Let $\tilde C$ be any connected set such that:

- $C \subseteq \tilde C$

Then $x \in \tilde C$.

Hence $\tilde C \in \CC_x$.

From Set is Subset of Union,

- $\tilde C \subseteq C$.

Hence $\tilde C = C$.

It follows that $C$ is a maximal connected set of $T$ by definition.

$\Box$

### Maximal Connected Set is Union of Connected Sets

Let $\tilde C$ be a maximal connected set of $T$ that contains $x$.

By definition:

- $\tilde C \in \CC_x$

From Set is Subset of Union:

- $\tilde C \subseteq C$

By maximality of $\tilde C$:

- $\tilde C = C$

$\blacksquare$

## Also see

## Sources

- 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.:
*Counterexamples in Topology*(2nd ed.) ... (previous) ... (next): Part $\text I$: Basic Definitions: Section $4$: Connectedness