arXiv subjects
Publications and source records attributed to Lucas Polymeris.
We construct a model of the Open Graph Axiom (OGA) in which there is a 2-entangled Suslin line $S$. Consequently, in this model, there is a 2-entangled uncountable linear order, but no such order is separable. This resolves a problem posed by Carroy, Levine, and Notaro \cite{carroy2025} and answers a question from McKenney on MathOverflow \cite{Mckenney2014}.
Let $T^*$ be an almost Suslin tree, that is, an Aronszajn tree with no stationary antichains. Krueger introduced a forcing axiom, $\mathrm{PFA}(T^*)$, for the class of proper forcings that preserve that $T^*$ is almost Suslin. He showed that $\mathrm{PFA}(T^*)$ implies several well-known consequences of the Proper Forcing Axiom ($\mathrm{PFA}$), including Suslin's Hypothesis and the P-ideal dichotomy. We extend this list by proving that $\mathrm{PFA}(T^*)$ also implies the Mapping Reflection Principle ($\mathrm{MRP}$) and the Open Graph Axiom ($\mathrm{OGA}$). Additionally, we show that $\mathrm{PFA}(T^*)$ implies that all special Aronszajn trees are club-isomorphic, but it does not imply that all almost Suslin trees are club-isomorphic.
A linear order $A$ is called strongly surjective if for every non empty suborder $B \preceq A$, there is an epimorphism from $A$ onto $B$ (denoted by $B \trianglelefteq A$). We show, answering some questions of D\'aniel T. Soukup, that under $\mathsf{MA}_{\aleph_{1}}$ there is a strongly surjective Countryman line. We also study the general structure of the class of Aronszajn lines under $\trianglelefteq$, and compare it with the well known embeddability relation $\preceq$. Under $\mathsf{PFA}$, the class of Aronszajn lines and the class of countable linear orders enjoy similar nice properties when viewed under the embeddability relation; both are well-quasi-ordered and have a finite basis. We show that this analogy does not extend perfectly to the $\trianglelefteq$ relation; while it is known that the countable linear orders are still well-quasi-ordered under $\trianglelefteq$, we show that already in $\mathsf{ZFC}$ the class of Aronszajn lines has an infinite antichain, and under $\mathsf{MA}_{\aleph_{1}}$ an infinite decreasing chain as well. We show that some of the analogy survives by proving that under $\mathsf{PFA}$, for some carefully constructed Countryman line $C$, $C$ and $C^{\star}$ form a $\trianglelefteq$-basis for the class of Aronszajn lines. Finally we show that this does not extend to all uncountable linear orders by proving that there is never a finite $\trianglelefteq$-basis for the uncountable real orders.