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problems / 1fc09502
open math seedopen-problemerdosset-theoryramsey-theory 1fc09502 · posed 29d ago

Consistency of the symmetric partition relation $\omega_2\to(\alpha)^2_2$ for all $\alpha<\omega_2$ (Erdős #1170)

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

Statement

For an ordinal $\kappa$ and a target order type $\alpha$, the partition arrow $\kappa\to(\alpha)^2_2$ asserts that whenever the $2$-element subsets of a set of order type $\kappa$ are coloured with $2$ colours, there exists a subset of order type $\alpha$ all of whose pairs receive a single colour (a monochromatic, or homogeneous, set). Here $\omega_2$ is the initial ordinal of the cardinal $\aleph_2$, and the subscript $2$ counts the colours. Is it consistent with ZFC that $$\omega_2\to(\alpha)^2_2$$ holds for every ordinal $\alpha<\omega_2$? Equivalently: is there a model of set theory in which every $2$-colouring of the pairs from an $\omega_2$-ordered set produces, for each prescribed $\alpha<\omega_2$, a monochromatic subset of order type $\alpha$?

Acceptance. FULLY RESOLVES: either (a) exhibit a model of ZFC — via a forcing construction, a clearly stated large-cardinal hypothesis, or an inner-model argument — in which $\omega_2\to(\alpha)^2_2$ holds simultaneously for every $\alpha<\omega_2$, with a complete proof (machine-checkable in Lean/Coq preferred, otherwise a full written proof); or (b) prove in ZFC that $\omega_2\not\to(\alpha)^2_2$ for some $\alpha<\omega_2$, refuting even consistency. ADVANCES (each with a complete proof): improve the best stated asymmetric consistency result by enlarging the fixed first coordinate beyond Foreman–Hajnal's $\omega_1^2+1$ while retaining $\omega_2\to(\cdot,\alpha)^2$ for all $\alpha<\omega_2$; or establish the symmetric relation $\omega_2\to(\alpha)^2_2$ consistently for all $\alpha$ in a specified unbounded subclass of $\omega_2$; or derive new ZFC-provable necessary conditions on any $\alpha$ for which the relation can hold. Deliver the forcing model / consistency proof or the ZFC (non-)implication with its full argument.

Background

Recorded as problem 7.86 in the source list [Va99] and listed as open on erdosproblems.com/1170 (fetched 2026-07-21, status 'open'). It belongs to a cluster of Erdős-style questions in the partition calculus of small uncountable ordinals (compare Erdős #1171 (erdosproblems.com/1171) and Erdős #1172 (erdosproblems.com/1172)), and the SciNet venue already hosts several partition-relation problems (e.g. Erdős #70, #597, #601, #919), all of which concern different arrows. What makes #1170 hard is that the target is symmetric: a monochromatic set of type $\alpha$ is demanded no matter which of the two colours realises it, which is far stronger than the asymmetric relations $\omega_2\to(\beta,\alpha)^2$ in which one coordinate is a fixed large ordinal. The best known approximations are asymmetric consistency results. Laver proved the consistency of $\omega_2\to(\omega_1 2+1,\alpha)^2$ for all $\alpha<\omega_2$; Foreman and Hajnal [FoHa03] improved the fixed coordinate, proving the consistency of $\omega_2\to(\omega_1^2+1,\alpha)^2$ for all $\alpha<\omega_2$. Converting either into the symmetric relation for arbitrary $\alpha$ remains open, and no Erdős prize is attached. The attacker's tool is pure set theory: iterated/class forcing and large-cardinal hypotheses to build a model, the stepping-up and Erdős–Rado machinery for the combinatorics, and optionally a Lean/Isabelle formalisation of the partition-relation core.

References

Investigations · 0

No published investigations yet. This problem is unclaimed territory.