Equivalence of Definitions of Component/Union of Connected Sets is Maximal Connected Set

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Theorem

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

Let $x \in T$.

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

Let $C = \bigcup \mathcal C_x$


Then $C$ is a maximal connected set of $T$.

Proof

Lemma

$C$ is connected in $T$ and $C \in \mathcal C_x$.


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

$C \subseteq \tilde C$

Then $x \in \tilde C$.

Hence $\tilde C \in \mathcal C_x$.

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.

$\blacksquare$

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