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Is the following true? Given a first countable Hausdorff space $X$ and a dense subset $Y\subset X$ which is initially $2^\omega$-initially Lindelöf in $X$. Let $F$ be a subset of $Y$ such that $\vert F\vert\leq 2^{\omega }$. Then $\big\vert \overline{F} \big\vert \leq 2^\omega $.

I actually want to prove something stronger, namely that $Y$ is Lindelöf in $X$. The proof goes by transfinite induction on $\alpha<\aleph_1$, one of the steps require the above statement to be true, so maybe some of the hypothesis above are not needed for that particular statement.

By Lindelöf in $X$, I mean that for any open cover $\gamma$ of $X$ there exists a countable subfamily $\eta\subset\gamma $ that covers $Y$. By $2^\omega$-initially Lindelöf I mean that for every open cover $\gamma$ of $X$ with $\vert \gamma \vert\leq 2^\omega$ there exists a countable subfamily $\eta\subset \gamma $ that covers $Y$.

This question has been cross-posted and answered in MathStackExchange Here is the link.

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  • $\begingroup$ $Y$ is Lindelöf in $X$ is just the same as $Y$ is Lindelöf as a subspace of $X$ (in its own right). $\endgroup$ May 20, 2020 at 10:31
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    $\begingroup$ @HennoBrandsma No, you can find a counter example in this paper $\endgroup$ May 20, 2020 at 18:26

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