English

Rado's Conjecture and the random algebra

Logic 2026-06-28 v1

Abstract

Rado's Conjecture (RC) is a compactness principle for a certain class of partial orders, namely trees TT of height ω1\omega_1 without cofinal branches, postulating that a partial order PP from this class can be decomposed into at most countably many antichains if and only if all its suborders of size ω1\omega_1 can be decomposed into at most countably many antichains. Rado's Conjecture is thus an uncountable version of Mirsky's theorem asserting that for every natural number nn, every infinite partial order PP can be decomposed into at most nn many antichains if and only if all its finite suborders can be decomposed into at most nn many antichains. Todorcevic showed that RC is consistent modulo a strongly compact cardinal. RC implies 2ωω22^\omega \le \omega_2, and has powerful consequences such as the Singular Cardinal Hypothesis, the failure of (κ)\square(\kappa) for every regular κω2\kappa \ge \omega_2 (and hence in particular the Projective Determinacy), and the Strong Chang Conjecture. It is also known that it is incompatible with Martin Axiom. We show that RC is consistent with 2ω=ω22^\omega = \omega_2 and the cardinal invariants in Cichon diagram corresponding to forcing with the random algebra, i.e., d=ω1\mathfrak{d} = \omega_1, cov(N)=ω2\mathrm{cov}(\mathcal{N}) = \omega_2, non(N)=ω1\mathrm{non}(\mathcal{N}) = \omega_1. This provides a new pattern of cardinal invariants known to be consistent with RC. To prove the theorem, we first observe that random algebras do not specialize non-special trees of height ω1\omega_1 without cofinal branches. Then we use the random algebra Bκ\mathcal{B}_\kappa for a strongly compact κ\kappa to define a new version of Mitchell forcing which yields the required result.

Cite

@article{arxiv.2606.29500,
  title  = {Rado's Conjecture and the random algebra},
  author = {Radek Honzik},
  journal= {arXiv preprint arXiv:2606.29500},
  year   = {2026}
}

Comments

15 pages

R2 v1 2026-07-22T20:14:35.688Z