English

Equivalences of promise compactness principles

Combinatorics 2026-05-28 v2

Abstract

For a pair of finite relational structures (A,B)(\mathfrak{A},\mathfrak{B}) such that A\mathfrak{A} homomorphically maps to B\mathfrak{B} we denote by K(A,B)K_{(\mathfrak{A},\mathfrak{B})} the following statement: for all structures I\mathfrak{I} with the same signature as A\mathfrak{A} if all finite substructures of I\mathfrak{I} homomorphically maps to A\mathfrak{A} then I\mathfrak{I} homomorphically maps to B\mathfrak{B}. In this article, we show that if (A,B)(\mathfrak{A},\mathfrak{B}) has no Ol\v{s}\'{a}k polymorphism, then K(A,B)K_{(\mathfrak{A},\mathfrak{B})} is equivalent to the ultrafilter principle over ZF\operatorname{ZF}. This includes the statements K(K3,K5)K_{(K_3,K_5)} and K(H2,Hc)K_{(H_2,H_c)} for all c2c\geq 2 where KnK_n denotes the clique of size nn and HkH_k denotes the ternary not-all-equal structure on a kk-element set. This means, for example, that in any ZF\operatorname{ZF} model, if every finitely 3-colourable graph can be coloured by 5 colours then all these graphs can in fact be coloured by 3 colours.

Keywords

Cite

@article{arxiv.2604.08365,
  title  = {Equivalences of promise compactness principles},
  author = {Bertalan Bodor},
  journal= {arXiv preprint arXiv:2604.08365},
  year   = {2026}
}