Simultaneously nonvanishing higher derived limits

TL;DR

This paper investigates conditions under which higher derived limits of inverse systems do not vanish simultaneously, revealing connections between set-theoretic assumptions and algebraic properties of inverse limits in topology and algebra.

Contribution

It proves that certain higher derived limits are nonzero under specific set-theoretic conditions, providing new insights into their behavior and answering a question by Bannister.

Findings

Higher derived limits $ eq 0$ for specific abelian groups and set-theoretic assumptions.

Vanishing of all higher limits implies large continuum cardinality.

Consistency results show simultaneous nonvanishing of multiple derived limits.

Abstract

The derived functors $\lim^n$ of the inverse limit find many applications in algebra and topology. In particular, the vanishing of certain derived limits $\lim^n \mathbf{A}[H]$, parametrized by an abelian group $H$, has implications for strong homology and condensed mathematics. In this paper, we prove that if $\mathfrak{d}=\omega_n$, then $\lim^n \mathbf{A}[H] \neq 0$ holds for $H=\mathbb{Z}^{(\omega_n)}$ (i.e. the direct sum of $\omega_n$-many copies of $\mathbb{Z}$). The same holds for $H=\mathbb{Z}$ under the assumption that $\mathrm{w}\diamondsuit(S^{k+1}_k)$ holds for all $k < n$. In particular, this shows that if $\lim^n \mathbf{A}[H] = 0$ holds for all $n \geq 1$ and all abelian groups $H$, then $2^{\aleph_0} \geq \aleph_{\omega+1}$, thus answering a question of Bannister. Finally, we prove some consistency results regarding simultaneous nonvanishing of derived limits, again in the case of $H = \mathbb{Z}$. In particular, we show the consistency, relative to $\mathsf{ZFC}$, of $\bigwedge_{2 \leq k < \omega} \lim^k \mathbf{A} \neq 0$.