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arXiv · 2401.15693

Squares, ultrafilters and forcing axioms

Chris Lambie-Hanson·Assaf Rinot·Jing Zhang
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Abstract

We study relationships between various set theoretic compactness principles, focusing on the interplay between the three families of combinatorial objects or principles mentioned in the title. Specifically, we show the following. (1) Strong forcing axioms, in general incompatible with the existence of indexed squares, can be made compatible with weaker versions of indexed squares. (2) Indexed squares and indecomposable ultrafilters with suitable parameters can coexist. As a consequence, the amount of stationary reflection known to be implied by the existence of a uniform indecomposable ultrafilter is optimal. (3) The Proper Forcing Axiom implies that any cardinal carrying a uniform indecomposable ultrafilter is either measurable or a supremum of countably many measurable cardinals. Leveraging insights from the preceding sections, we demonstrate that the conclusion cannot be improved.

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Chris Lambie-Hanson, Assaf Rinot, Jing Zhang. 2024-01-28. Squares, ultrafilters and forcing axioms. https://arxiv.org/abs/2401.15693

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