For any set $X$ and cardinal $\kappa$ let $[X]^\kappa$ be the collection of all subsets of $X$ of cardinality $\kappa$.


I was looking for $T\_2$-spaces $(X,\tau)$ with the property that 



> 
> $(P)$ there is an injective function $f:[X]^\omega\to \tau$ such that for all $s\in [X]^\omega$ we have $s\subseteq f(s)$. 
> 
> 
> 


**Question.** If $(X,\tau)$ is infinite, Hausdorff, and non-separable, is there a function with the properties described in $(P)$?


**Note.** I had the following remark in the original version of this post, but KP Hart made me aware that it is false - thanks for spotting my mistake!


[False] Obviously, if $(X,\tau)$ is [separable](https://en.wikipedia.org/wiki/Separable_space) and contains at least two different countable dense subsets $s$, $s'$, then the only member of $\tau$ that contains $s$ or $s'$ is $X$ itself, so there cannot be an injective function $f$ as described in $(P)$. [/False]



Injective choice function for non-separable $T_2$-spaces
Here is a partial answer: if $|X|^{\aleph\_0}=|X|$ then the answer is yes.
First take an injective function $F:[X\times X]^\omega\to X$ and then take some
function $G:[X\times X]^\omega\to X$ such that for all $A\in[X\times X]^\omega$ the point $\langle F(A),G(A)\rangle$ is not in $A$.
Translate this via a bijection between $X$ and $X\times X$ to a function $H:[X]^\omega\to X$ such that $H(A)\notin A$ for all $A$.


Then $f:A\mapsto X\setminus\{H(A)\}$ is as required.


Note that this uses nothing but the $T\_1$-property and works even for the co-finite topology.


