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Logic Seminar

Sebastian JungeCornell University
Partitions and simultaneously vanishing higher derived limits, part 2

Monday, September 11, 2023 - 2:55pm
Malott 205

Let A be the inverse system of abelian groups Ax=n=0i<x(n)Z, for xNN, equipped with the canonical projection maps. The higher derived limits lim appear in Mardesic and Prasolov's calculations of the strong homology groups of a countable topological sum of d-dimensional Hawaiian earrings. In particular, in order for strong homology to be additive for the class of locally compact second countable spaces \lim^s \mathbf{A} must vanish for all s. While Mardesic and Prasolov showed that \lim^1 \mathbf{A} \ne 0 if the Continuum Hypothesis holds, Bergfalk and Lambie-Hanson recently proved the consistency, relatively to a weakly compact cardinal, of \lim^s \mathbf{A} = 0 for all s. We will show that Bergfalk and Lambie-Hanson's result can be viewed as a combination of a consistency proof of a partition hypothesis and a ZFC proof that this partition hypothesis implies \lim^s \mathbf{A} = 0 for all s. This is a multipart talk: part 1 will give an introduction and overview of the results, part 2 will establish the consistency result, and part 3 will derive \lim^s \mathbf{A} from the partition hypothesis. The result being presented is due to Bannister, Bergfalk, Moore, and Todorcevic.