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Non-vanishing higher derived limits

Published 8 Jul 2021 in math.LO and math.CT | (2107.03787v1)

Abstract: In the study of strong homology Marde\v{s}i\'c and Prasolov isolated a certain inverse system of abelian groups A\mathbf A indexed by elements of ω<sup>ω\omega<sup>\omega. They showed that if strong homology is additive on a class of spaces containing closed subsets of Euclidean spaces then the higher derived limits lim<sup>n</sup>A\lim<sup>n</sup> \mathbf A must vanish, for $n&gt;0$. They also proved that under the Continuum Hypothesis lim<sup>1</sup>A0\lim<sup>1</sup> \mathbf A \neq 0. The question whether lim<sup>n</sup>A\lim<sup>n</sup> \mathbf A vanishes, for $n&gt;0$, has attracted considerable interest from set theorists. Dow, Simon and Vaughan showed that under PFA lim<sup>1</sup>A=0\lim<sup>1</sup> \mathbf A =0. Bergfalk show that it is consistent that lim<sup>2</sup>A\lim<sup>2\mathbf</sup> A does not vanish. Later Bergfalk and Lambie-Hanson showed that, modulo a weakly compact cardinal, it is relatively consistent with ZFC that lim<sup>n</sup>A=0\lim<sup>n</sup> \mathbf A =0, for all nn. The large cardinal assumption was recently removed by Bergfalk, Hru\v{s}ak and Lambie-Henson. We complete the picture by showing that, for any $n&gt;0$, it is relatively consistent with ZFC that lim<sup>n</sup>A0\lim<sup>n</sup> \mathbf A \neq 0.

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