Lindelöf Property is Preserved under Continuous Surjection

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

Let $\phi: T_A \to T_B$ be a continuous surjection.

If $T_A$ is a Lindelöf space, then $T_B$ is also a Lindelöf space.

Proof
Let $T_A$ be a Lindelöf space.

Take an open cover $\mathcal U$ of $T_B$.

From Preimage of Cover is Cover, $\left\{{\phi^{-1} \left({U}\right): U \in \mathcal U}\right\}$ is a cover of $X_A$.

But $\phi$ is continuous, and for all $U \in \mathcal U$, $U$ is open in $T_B$.

It follows that $\forall U \in \mathcal U: \phi^{-1} \left({U}\right)$ is open in $T_A$.

So $\left\{{\phi^{-1} \left({U}\right):\ U \in \mathcal U}\right\}$ is an open cover of $T_A$.

$T_A$ is Lindelöf, so we take a countable subcover:
 * $\left\{{\phi^{-1} \left({U_1}\right), \ldots, \phi^{-1} \left({U_n}\right),\ldots}\right\}$

We have that $\phi$ is surjective.

So from Surjection iff Right Inverse:
 * $\phi \left({\phi^{-1} \left({A}\right)}\right) = A$

So:
 * $\left\{{\phi\left({\phi^{-1}\left({U_1}\right)}\right), \ldots, \phi \left({\phi^{-1} \left({U_n}\right)}\right)},\ldots\right\} = \left\{{U_1, \ldots, U_n},\ldots\right\} \subseteq \mathcal U$

is a countable subcover of $\mathcal U$ on $T_B$.

Also see

 * Compactness Properties Preserved under Continuous Surjections