# Modified Fort Space is not T3, T4 or T5

## Theorem

Let $T = \left({S, \tau_{a, b}}\right)$ be a modified Fort space.

Then $T$ is not a $T_3$ space, $T_4$ space or $T_5$ space.

## Proof

We have that a Modified Fort Space is $T_1$.

From $T_1$ Space is $T_0$ Space, $T$ is a $T_0$ space.

We then have that a Modified Fort Space is not $T_2$.

From Regular Space is $T_2$ Space, $T$ is not a regular space.

By definition, a regular space is a space that is both a $T_0$ space and a $T_3$ space.

But $T$ is a $T_0$ space and not a regular space.

So it follows that $T$ can not be a $T_3$ space.

Next we have that a Normal Space is a $T_3$ Space.

But as $T$ is not a $T_3$ space, $T$ can not be a normal space.

By definition, a normal space is a space that is both a $T_1$ space and a $T_4$ space.

But $T$ is a $T_1$ space and not a normal space.

So it follows that $T$ can not be a $T_4$ space.

Finally we have that a $T_5$ Space is a $T_4$ Space.

But as $T$ is not a $T_4$ space, $T$ can not be a $T_5$ space.

$\blacksquare$

## Sources

- 1970: Lynn Arthur Steen and J. Arthur Seebach, Jr.:
*Counterexamples in Topology*... (previous) ... (next): $\text{II}: \ 27: \ 3$