# Cover of Doubletons of Infinite Particular Point Space has no Locally Finite Refinement

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## Theorem

Let $T = \left({S, \tau_p}\right)$ be an infinite particular point space.

Let $\mathcal C$ be the open cover of $T$ defined as:

- $\mathcal C = \left\{{\left\{{x, p}\right\}: x \in S, x \ne p}\right\}$

Then $\mathcal C$ has no open refinement which is locally finite.

## Proof

Suppose $T$ is an infinite particular point space.

As $S$ is infinite, $\mathcal C$ is also infinite.

Let $x \in S, x \ne p$.

Then any neighborhood of $x$ must contain $p$, by the nature of the particular point topology.

But $p$ is contained in all elements of $\mathcal C$.

That is:

- $\forall C \in \mathcal C: p \in C$

So any neighborhood of $x$ intersects with all elements of the open cover $\mathcal C$.

As $\mathcal C$ is infinite, it therefore has no open refinement which is locally finite.

$\blacksquare$

## Sources

- 1970: Lynn Arthur Steen and J. Arthur Seebach, Jr.:
*Counterexamples in Topology*... (previous) ... (next): $\text{II}: \ 9 - 10: \ 16$