T1 Space is Preserved under Closed Bijection

Theorem
Let $T_A = \left({X_A, \vartheta_A}\right), T_B = \left({X_B, \vartheta_B}\right)$ be topological spaces.

Let $\phi: T_A \to T_B$ be a closed bijection.

If $T_A$ is a $T_1$ (Fréchet) space, then so is $T_B$.

Proof
Let $T_A$ be a $T_1$ (Fréchet) space.

By definition, all points in $T_A$ are closed.

Let $a \in X_A$.

Then $\left\{{a}\right\}$ is a closed set.

As $\phi$ is a closed mapping it follows directly that $\phi \left({\left\{{a}\right\}}\right)$ is closed.

As $\phi$ is a bijection it follows that every point in $X_B$ is the image under $\phi$ of a single point in $X_A$.

Hence every point in $X_B$ is closed.

That is, $T_B$ is a $T_1$ (Fréchet) space.