# Basis Test for Isolated Point

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## Theorem

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

Let $\BB$ be a synthetic basis of $T$.

Let $H \subseteq S$.

Then $x \in H$ is an isolated point of $H$ if and only if:

- $\exists U \in \BB : U \cap H = \set {x}$

## Proof

### Necessary Condition

Let $x \in H$ be an isolated point of $H$.

By definition of an isolated point:

- $\exists U \in \tau: U \cap H = \set x$

By definition of a synthetic basis of $T$:

- $\exists V \in \BB: x \in V \subseteq U$

From Set Intersection Preserves Subsets:

- $V \cap H \subseteq U \cap H = \set x$

From Singleton of Element is Subset:

- $\set x \subseteq V \cap H$

From set equality:

- $V \cap H = \set x$

$\Box$

### Sufficient Condition

Let $U \in \BB : U \cap H = \set {x}$.

By definition of synthetic basis of $T$:

- $U \in \tau$

Then $x$ is an isolated point of $H$ by definition.

$\blacksquare$