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Let $\mathcal U$ be a free ultrafilter on a set $X$. For $n\in\mathbb N$ let $\mathcal F$ be a family of $n$-element subsets of $X$ such that $\bigcup\mathcal F\in\mathcal U$.

Question. Is there a set $U\in\mathcal U$ and a subfamily $\mathcal E\subset\mathcal F$ such that $\bigcup\mathcal E\in\mathcal U$ and $|U\cap E|=1$ for every $E\in\mathcal E$?

I do not know the answer even for $n=2$ and countable set $X$.

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Here's a try for $n=3$ which I think generalizes to any $n$.

Well-order $\mathcal{F}$ as $\{F_\alpha\}$. By discarding any $F_\alpha$ which is contained in $\bigcup_{\beta <\alpha} F_\beta$, we can ensure that each $F_\alpha$ contains at least one point which is not in any previous $F_\beta$, without affecting $U = \bigcup F_\alpha$.

We can now inductively construct a set $V \subset U$ with the property that $|V \cap F_\alpha|= 1$ or $2$ for all alpha. At each $\alpha$ at least one element of $F_\alpha$ is new and we can include it or not to ensure the condition for $F_\alpha$.

Now $U \setminus V$ has the same property, so since $U \in \mathcal{U}$, wlog we can assume $V \in \mathcal{U}$.

Let $\mathcal{E}_1 \subseteq \mathcal{F}$ consist of those $F_\alpha$ which intersect $V$ in exactly one point, and let $\mathcal{E}_2$ consist of the other $F_\alpha$, which all intersect $V$ in two points. If the union of $\mathcal{E}_1$ belongs to $\mathcal{U}$ then we are done. Otherwise the union of $\mathcal{E}_2$ belongs to $\mathcal{U}$ and this reduces us to the $n=2$ case.

The general reduction turns the problem for an arbitrary $n$ into the same problem for some smaller $n$.

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  • $\begingroup$ So nice argument! Thank you for the help. $\endgroup$ – Taras Banakh Oct 11 '18 at 19:58
  • $\begingroup$ You are welcome! $\endgroup$ – Nik Weaver Oct 11 '18 at 20:09
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For $n=2$ and any $X$: for each $x\in \cup \mathcal F$ fix any edge $(x,y)\in \mathcal F$ and draw an arrow from $x$ to $y$. Remove all other edges. Remaining edges still have the same union as $\mathcal{F}$, but they form a directed graph with outdegrees at most 1. Such a graph has a proper 3-coloring (it suffices to prove this for finite subgraphs by compactness theorem, and for them the result is well known and easy: remove the vertex with minimal in-degree, it is at most 1, and proceed by induction). One of three colors works.

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    $\begingroup$ You can alternatively take a covering forest of the graph (which constructs $\mathcal{E}$), and then you have a 2-coloring (fix 1 vertex of some color in each component, and elements of a single component have the same color iff they're at even distance), and then take the set of vertices of some fixed color. $\endgroup$ – YCor Oct 11 '18 at 18:58
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    $\begingroup$ It's just a maximal subset of edges with containing no loop. $\endgroup$ – YCor Oct 11 '18 at 19:07
  • $\begingroup$ Thank you for the answers. What about the case $n\ge 3$? (Starting from $n=3$ I have some applications to the problem of normality of hyperballeans). $\endgroup$ – Taras Banakh Oct 11 '18 at 19:42
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    $\begingroup$ My argument with covering forest does not work, I think. Instead, one consider a maximal non-covering subfamily of $\mathcal{E}$, so it has the same union: then the corresponding graph has the property that no edge joins two vertices of valency $\ge 2$. It follows that each of its component is a tree, with one vertex joined to all others. $\endgroup$ – YCor Oct 11 '18 at 19:55
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Consider this with assuming that $\mathcal{F}$ consists of finite subsets of bounded cardinal; let $n$ be the max of these cardinals and let us argue by induction on $n$. The goal is to find $\mathcal{E}\subset\mathcal{F}$ and $U\in\mathcal{U}$ such that $U\subset\bigcup\mathcal{E}$ and and $|E\cap U|\le 1$ for every $E\in\mathcal{E}$.

First, consider a maximal subset $\mathcal{E}$ of the cover such that no element is in the union of others (we call this ``minimal"). So $V:=\bigcup\mathcal{F}=\bigcup\mathcal{E}$. Write $V=V_1\cup V_2$, where $V_2$ is the set of elements of $V$ that belong to at least two elements of $\mathcal{E}$. The minimality of $\mathcal{E}$ implies that no $E\in\mathcal{E}$ is contained in $V_2$ (i.e., has nonempty intersection with $V_1$).

Since the intersection of elements of $\mathcal{E}$ with $V_1$ are pairwise equal or disjoint, we can partition $V_1$ into $n$ subsets each intersecting, each element of $\mathcal{E}$ in at most a singleton. Hence we can conclude if $V_1\in\mathcal{U}$ (with $U$ being one of those $n$ subsets of $V_1$).

Otherwise, $V_2\in\mathcal{U}$. Since $|E\cap V_2|\le n-1$ for all $E\in\mathcal{E}$, we can conclude by induction (and find $U\subset V_2$).

PS Nik Weaver posted an answer while I was writing this one, but I still post, although I guess it's the same idea.

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  • $\begingroup$ Thank you, Yves for the answer (in fact, MO is incredibly efficient). $\endgroup$ – Taras Banakh Oct 11 '18 at 22:44

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