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Suppose there exists a subset of $\Bbb R$ which has cardinality $\omega_1$. Is it then necessarilly true that for every collection of $\omega_1$ subsets of $\Bbb R$ there exists a choice function?

I suppose that the answer for the above question is no, just like existence of countable subsets of $\Bbb R$ doesn't imply countable choice for $\Bbb R$. However, I suppose, every proof would either need explicit use of forcing, or a model of ZF which already has some weird properties, like amorphous sets or something.

Note that, obviously, we assume that AC doesn't necessarilly hold.

I added "forcing" tag, because I believe this is what answer requires. If it's not the case, feel free to remove it.

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    $\begingroup$ Can someone explain why I am wrong to think that $\mathbb{R}$ always has a subset of cardinality $\omega_1$? $\endgroup$ Commented Oct 5, 2014 at 20:25
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    $\begingroup$ Andrej, without some choice, you cannot always build an $\omega_1$-sequence of distinct reals (if you try to build one, you will find that you are using AC). Indeed, AD implies outright that there is no such sequence. $\endgroup$ Commented Oct 5, 2014 at 20:26
  • $\begingroup$ @Andrej: Truss showed that by collapsing all the ordinals below a limit cardinal (larger than the continuum) to be countable, and taking a symmetric submodel, you have a model of $\sf ZF$ in which all subsets of $\Bbb R$ are either countable or have a perfect subset, in particular $\aleph_1\nleq2^{\aleph_0}$ in such model. $\endgroup$
    – Asaf Karagila
    Commented Oct 5, 2014 at 20:29
  • $\begingroup$ Indeed, my brain is running in circles trying to get $\omega_1$ embedded in $\mathbb{R}$, or $\mathcal{P}(\omega)$, or $\mathbb{R} \times \mathbb{R}$, or some such. Intriguing. $\endgroup$ Commented Oct 5, 2014 at 20:29
  • $\begingroup$ @Joel: That is correct, but $\sf AD$ is a consistency overkill. This can be achieved within the confines of $\operatorname{Con}({\sf ZF})$. $\endgroup$
    – Asaf Karagila
    Commented Oct 5, 2014 at 20:30

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No, since there is a surjection from $\Bbb R$ onto $\omega_1$, there is always an injection from $\omega_1$ into $\mathcal P(\Bbb R)$; but it's not difficult to arrange that there is no choice function for some sequence of subsets like that.

To see that the range of the injection need not have a choice function, simply consider a construction similar to Cohen's first model, only now add $\omega_1$ Cohen reals (and take permutations moving them, etc.). In this model there is a Dedekind-finite set of reals which can be mapped onto $\omega_1$, which defines a sequence of $\omega_1$ sets of reals by taking preimages of each point, but there is no choice function for even countably many of these, since that would show that the set is not Dedekind-finite.


You can in fact get a similar result in Cohen's first model. If $A\subseteq\Bbb R$ is the Dedekind-finite set of generic reals, then $A$ can be mapped onto $\omega$, pick $A_n$ to be the preimage of $n$ under some fixed surjection; and take $A_\alpha=\{x_\alpha\}$ to be the $\alpha$-th real of the ground model ($L$). Then the $A_n$'s don't admit a choice function, so of course the full collection doesn't.

The modification suggested with adding $\omega_1$ Cohen reals can be modified, by taking the filter of subgroups defined by countable supports rather than finite; in the resulting model $\sf DC$ should hold (this should be, I think, $L(\Bbb R)$, which satisfies $\sf DC$). So we have a model that every countable family of sets of reals admits a choice function, but there is a family of size $\aleph_1$ which does not admit a choice function.

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  • $\begingroup$ Ahh, right. A subset of size $\omega_1$ doesn't do anything when we add Cohen reals. Thanks. $\endgroup$
    – Wojowu
    Commented Oct 5, 2014 at 20:27

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