Let $\kappa>\omega$ be a cardinal. We say that ${\cal A}\subseteq{\cal P}(\kappa)$ has the *finite intersection property* (FIP) if $|A|=\kappa$ for $A\in{\cal A}$, and $|A\cap B|<\aleph\_0$ for $A\neq B\in{\cal A}$.


For which cardinals $\kappa>\omega$ is there a family with FIP ${\cal A}\subseteq {\cal P}(\kappa)$ such that $|{\cal A}|>\kappa$?



Families with finite intersection property on $\kappa>\omega$
This is a question that connects to many things in set theory (and they are sometimes called ``strongly almost disjoint families").


First, an old result of Baumgartner (see Section 6 of [1]) shows that you can start with a model of GCH and force the existence of such families for a given $\kappa$ without collapsing cardinals or changing cofinalities. Baumgartner's forcing will result in a model in which $\kappa\leq 2^{\aleph\_0}$. He obtains more general results: Assume GCH. Given infinite cardinals $\nu<\kappa<\lambda$ with $\nu$ regular, you can force the existence of a family $\mathcal{A}\subseteq [\kappa]^{\kappa}$ of size $\lambda$ such that any two members of $\mathcal{A}$ have intersection of cardinality less than $\nu$.


On the other hand, ZFC tells us that there are no such $\kappa$ above $\beth\_\omega$, the first strong limit singular cardinal (best viewed as the supremum of the sequence $2^{\aleph\_0}, 2^{2^{\aleph\_0}}, 2^{2^{2^{\aleph\_0}}},..$).


If we let $\mu=\beth\_\omega$, then by Shelah's Revised GCH Theorem [3], for any $\kappa>\mu$ we can find $\mathcal{P}\subseteq [\kappa]^{<\mu}$ and a regular $\sigma<\mu$ such that


* $|\mathcal{P}|=\kappa$, and
* every $A\in [\kappa]^{<\mu}$ is a union of fewer than $\sigma$ members of $\mathcal{P}$.


(See in particular Conclusion 1.2 (4) from [3].)


Since $\sigma$ is regular, each subset of $\kappa$ of cardinality at least $\sigma$ must have a subset of cardinality at least $\sigma$ lying in $\mathcal{P}$. Thus, given $\mathcal{A}\subseteq[\kappa]^\kappa$ of cardinality $\kappa^+$, we can find $X\subseteq \kappa$ of cardinality $\sigma$ such that
$$|\{A\in\mathcal{A}: X\subseteq A\}|=\kappa^+,$$
which is much stronger than you require.


Now what happens for $\kappa$ between $2^{\aleph\_0}$ and $\beth\_\omega$? This turns out to be connected to pcf theory, and is very much open.


For example, for a cardinal $\kappa>2^{\aleph\_0}$, the following two statements are equivalent:


1. There is a family $\mathcal{A}\subseteq [\kappa]^{\aleph\_1}$ of cardinality $>\kappa$ with pairwise finite intersection.
2. There is a sequence $\langle A\_\alpha:\alpha<\omega\_1\rangle$ such that each $A\_\alpha$ is a finite collection of regular cardinals from the interval $(2^{\aleph\_0},\kappa]$, and such that for every infinite $X\subseteq \omega\_1$, ${\rm maxpcf}(\bigcup\_{\alpha\in X}A\_\alpha)>\kappa$.


The above is a theorem of Shelah (see Section 6 of [2]). It is unknown if this is consistent, but the existence of such a family entails some drastic failures of the singular cardinals hypothesis.


[1] *Baumgartner, James E.*, [**Almost-disjoint sets, the dense set problem and the partition calculus**](http://dx.doi.org/10.1016/0003-4843(76)90018-8), Ann. Math. Logic 9, 401-439 (1976). [ZBL0339.04003](https://zbmath.org/?q=an:0339.04003).


[2] *Shelah, Saharon*, [**More on cardinal arithmetic**](http://dx.doi.org/10.1007/BF01270465), Arch. Math. Logic 32, No. 6, 399-428 (1993). [ZBL0799.03052](https://zbmath.org/?q=an:0799.03052).


[3] *Shelah, Saharon*, [**The generalized continuum hypothesis revisited**](http://dx.doi.org/10.1007/BF02773223), Isr. J. Math. 116, 285-321 (2000). [ZBL0955.03054](https://zbmath.org/?q=an:0955.03054).


