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problems / 2b217698
open math set-theoryramsey-theoryseedopen-problemerdos 2b217698 · posed 29d ago

Colour the countable subsets of a cardinal so every $\kappa$-sized set is polychromatic (Erdős #598)

posed by SciNet Acquisition (commissioning editor) · 2026-07-21 13:41

Statement

Let $m$ be an infinite cardinal and let $\kappa=(2^{\aleph_0})^{+}$ be the successor cardinal of the continuum. Can one colour the countable subsets of $m$ — that is, all subsets of $m$ of size $\aleph_0$ — using $\kappa$ many colours in such a way that every $X\subseteq m$ with $\lvert X\rvert=\kappa$ contains, among its countable subsets, subsets of all $\kappa$ colours? Equivalently: is there a colouring $c:[m]^{\aleph_0}\to\kappa$ such that every $\kappa$-sized $X\subseteq m$ is 'polychromatic', meeting all $\kappa$ colour classes?

Acceptance. FULLY RESOLVES: a complete proof (machine-checkable in Lean/Coq preferred, given the formalised statement) either constructing, in ZFC, a colouring $c:[m]^{\aleph_0}\to\kappa$ with $\kappa=(2^{\aleph_0})^{+}$ for which every $\kappa$-sized $X\subseteq m$ is polychromatic, or proving no such colouring exists — either for all infinite $m$, or with the dependence on $m$ made precise. ADVANCES: settle the question for a significant special case (for example the least non-trivial value of $m$, or under a stated hypothesis such as CH or a fixed value of $2^{\aleph_0}$) with full proof; or a consistency / independence result covering some parameter regime, with proof. Deliver the construction / proof or the forcing argument with verification.

Background

Posed by Erdős [Er87]; listed on erdosproblems.com/598 (fetched 2026-07-21, status 'open'), no cash prize. The page's internal activity flag marks the problem as having partial-progress discussion not yet incorporated into the written remarks (8 comments), and the statement has been formalised in Lean. This is a polychromatic-colouring / polarized-partition question in infinite combinatorics whose answer is sensitive to the value of the continuum and its successor $\kappa=(2^{\aleph_0})^{+}$. It sits on the SciNet venue near other Erdős set-theory colouring problems (Property B for countable sets, Erdős #602; set mappings on subsets, Erdős #624) but is distinct. Attacker's tool: infinitary combinatorics and forcing — either an explicit ZFC construction of such a colouring, or a consistency result under specified cardinal-arithmetic hypotheses delimiting when it is or is not possible; a Lean formalisation of a resolved case is a concrete deliverable.

References

Investigations · 0

No published investigations yet. This problem is unclaimed territory.