Ladders and squares

In 1984, Ditor asked two questions: (1) For each $n\inω$ and infinite cardinal $κ$, is there a join-semilattice of breadth $n+1$ and cardinality $κ^{+n}$ whose principal ideals have cardinality $< κ$? (2) For each $n \in ω$, is there a lower-finite lattice of cardinality $\aleph_{n}$ whose elements have at most $n+1$ lower covers? We show that both questions have positive answers under the axiom of constructibility, and hence consistently with $\mathsf{ZFC}$. More specifically, we derive the positive answers from assuming that $\square_κ$ holds for enough $κ$'s.

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Publication Details

Journal
Advances in Mathematics
Published
2025-11-27
DOI
https://doi.org/10.1016/j.aim.2025.110714
Primary Topic
Advanced Topology and Set Theory
Type
article
Field-Weighted Citation Impact
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article

Ladders and squares

Lorenzo Notaro
Advances in Mathematics
Advanced Topology and Set Theory
article

Ladders and squares

Lorenzo Notaro
article en

Abstract

In 1984, Ditor asked two questions: (1) For each $n\inω$ and infinite cardinal $κ$, is there a join-semilattice of breadth $n+1$ and cardinality $κ^{+n}$ whose principal ideals have cardinality $< κ$? (2) For each $n \in ω$, is there a lower-finite lattice of cardinality $\aleph_{n}$ whose elements have at most $n+1$ lower covers? We show that both questions have positive answers under the axiom of constructibility, and hence consistently with $\mathsf{ZFC}$. More specifically, we derive the positive answers from assuming that $\square_κ$ holds for enough $κ$'s.

Advances in MathematicsVol. 485
Openalex Percentile: Top 57%
Advanced Topology and Set Theory
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Ladders and squares — Lorenzo Notaro · Advances in Mathematics (2025) | TGRS Research Map | TGRS