# Particular Point Topology with Three Points is not T4

## Contents

## Theorem

Let $T = \struct {S, \tau_p}$ be a particular point space such that $S$ is not a singleton or a doubleton.

That is, such that $S$ has more than two distinct elements.

Then $T$ is not a $T_4$ space.

## Proof

We have that there are at least three elements of $S$.

So, consider $x, y, p \in S: x \ne y, x \ne p, y \ne p$.

Then $X = \set x, Y = \set y$ are closed in $T$ and $X \cap Y = \O$.

Suppose $U, V \in \tau_p$ are open sets in $T$ such that $X \subseteq U, Y \subseteq V$.

But as $p \in U, p \in V$ we have that $U \cap V \ne \O$.

So $T$ is not a $T_4$ space.

$\blacksquare$

## Also see

## Mistakes in Sources

### Non-Trivial Particular Point Topology is not T4

See 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.: *Counterexamples in Topology* (2nd ed.): Part $\text {II}$: Counterexamples: $8 \text { - } 10: \ 4$ where it is stated that:

*Every particular point topology is $T_0$, but since there are no disjoint open sets, none of the higher separation axioms are satisfied unless $X$ has only one point.*

## Sources

- 1978: Lynn Arthur Steen and J. Arthur Seebach, Jr.:
*Counterexamples in Topology*(2nd ed.) ... (previous) ... (next): Part $\text {II}$: Counterexamples: $8 \text { - } 10$. Particular Point Topology: $4$