Problem Statement
Let $X$ be a set of cardinality $\aleph_\omega$ and $f$ be a function from the finite subsets of $X$ to $X$ such that $f(A)\not\in A$ for all $A$. Must there exist an infinite $Y\subseteq X$ that is independent - that is, for all finite $B\subset Y$ we have $f(B)\not\in Y$?
Categories:
Set Theory
Progress
A problem of Erdős and Hajnal [ErHa58], who proved that if $\lvert X\rvert <\aleph_\omega$ then the answer is no. Erdős suggests in [Er99] that this problem is 'perhaps undecidable'.Source: erdosproblems.com/623 | Last verified: January 15, 2026