# Either-Or Topology is not Locally Arc-Connected

## Theorem

Let $T = \left({S, \tau}\right)$ be the either-or space.

Then $T$ is not a locally arc-connected space.

## Proof

Let $\mathcal B$ be a basis for $\tau$.

Suppose that $0 \in B$, where $B \in \mathcal B$.

Then by definition of the either-or topology, $\left({-1 \,.\,.\, 1}\right) \subseteq B$.

In particular, $\dfrac 1 2 \in B$.

Let $f: \left[{0 \,.\,.\, 1}\right] \to B$ be any injection such that:

- $f \left({0}\right) = 0$
- $f \left({1}\right) = \dfrac 1 2$

Since $0 \notin \left\{{\dfrac 1 2}\right\}$, it follows that $\left\{{\dfrac 1 2}\right\}$ is open in $\tau$.

As $f$ is an injection, it follows that:

- $f^{-1} \left({\left\{{\dfrac 1 2}\right\}}\right) = \left\{{1}\right\}$

By Closed Real Interval is not Open Set, $\left\{{1}\right\} = \left[{1 \,.\,.\, 1}\right]$ is not open in $\left[{0 \,.\,.\, 1}\right]$.

Thus, $f$ cannot be continuous.

Hence, no arc from $0$ to $\dfrac 1 2$ can exist.

It follows that $B$ is not arc-connected, and hence $T$ is not locally arc-connected.

$\blacksquare$

## Sources

- 1970: Lynn Arthur Steen and J. Arthur Seebach, Jr.:
*Counterexamples in Topology*... (previous) ... (next): $\text{II}: \ 17: \ 4$