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If $\kappa$ is a cardinal and $X$ is a set, let $[X]^\kappa$ denote the collection of subsets of $X$ that have cardinality $\kappa$.

Let $\beta>\omega$ and $\beta \leq 2^{\omega}$. Is there ${\cal C}\subseteq [\mathbb{R}]^\beta$ such that every member of $[\mathbb{R}]^\omega$ is contained in exactly one member of ${\cal C}$?

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  • $\begingroup$ Let B and D be distinct infinite subsets. Then B union D must live in the same member of C as both B and D. So any superset of B must be in the same member as B. This includes the whole space, when it fits. (As Nik Weaver observed in a now deleted comment.) Gerhard "Infinity Is A Strange Place" Paseman, 2019.09.05. $\endgroup$ Sep 6, 2019 at 5:14

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Suppose that every countably infinite subset of $\mathbb R$ is contained in exactly one member of $\mathcal C$, where $\mathcal C\subseteq\mathcal P(\mathbb R)$ and $\mathbb R\notin\mathcal C$. Let $A$ be a countably infinite subset of $\mathbb R$. Choose a set $S\in\mathcal C$ such that $A\subseteq S$, and choose an element $t\in\mathbb R\setminus S$. Consider the countably infinite set $B=A\cup\{t\}$. Either $B$ is contained in no member of $\mathcal C$, or else $A$ is contained in two different members of $\mathcal C$; either way we have a contradiction.

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If every set in $[\mathbb{R}]^{\aleph_0}$ is contained in a unique member of $\mathcal{F}$, then by induction on $\aleph_0 \leq \kappa \leq \mathfrak{c}$, it is easy to see that every set in $[\mathbb{R}]^{\kappa}$ is contained in a unique member of $\mathcal{F}$. It follows that $\mathbb{R} \in \mathcal{F}$ and hence $\mathcal{F} = \{\mathbb{R}\}$.

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  • $\begingroup$ Nice argument, msybe a little too terse. Let's see. Suppose $\kappa\gt\omega$, $A\in[\mathbb R]^\kappa$, and every infinite subset of $\mathbb R$ of size $\lt\kappa$ is contained in a unique member of $\mathcal C$. Write $A=\bigcup_{\alpha\lt\kappa}A_\alpha$ where $\alpha\lt\beta\implies A_\alpha\lt A_\beta$ and $\aleph_0\le|A_\alpha|\lt\kappa$. Then each $A_\alpha$ is contained in a unique $B_\alpha\in\mathcal C$, so the $B_\alpha$ are all the same, so $A$ is contained in a unique member of $\mathcal C$. Right? $\endgroup$
    – bof
    Sep 6, 2019 at 21:08

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