Equivalences of promise compactness principles
Combinatorics
2026-05-28 v2
Abstract
For a pair of finite relational structures such that homomorphically maps to we denote by the following statement: for all structures with the same signature as if all finite substructures of homomorphically maps to then homomorphically maps to . In this article, we show that if has no Ol\v{s}\'{a}k polymorphism, then is equivalent to the ultrafilter principle over . This includes the statements and for all where denotes the clique of size and denotes the ternary not-all-equal structure on a -element set. This means, for example, that in any 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}
}