English

Lim colim versus colim lim. II: Derived limits over a pospace

Algebraic Topology 2022-11-21 v2 Geometric Topology

Abstract

\v{C}ech cohomology Hn(X)H^n(X) of a separable metrizable space XX is defined in terms of cohomology of its nerves (or ANR neighborhoods) PβP_\beta whereas Steenrod-Sitnikov homology Hn(X)H_n(X) is defined in terms of homology of compact subsets KαXK_\alpha\subset X. We show that one can also go vice versa: in a sense, Hn(X)H^n(X) can be reconstructed from Hn(Kα)H^n(K_\alpha), and if XX is finite dimensional, Hn(X)H_n(X) can be reconstructed from Hn(Pβ)H_n(P_\beta). The reconstruction is via a Bousfield-Kan/Araki-Yoshimura type spectral sequence, except that the derived limits have to be "corrected" so as to take into account a natural topology on the indexing set. The corrected derived limits coincide with the usual ones when the topology is discrete, and in general are applied not to an inverse system but to a "partially ordered sheaf". The "correction" of the derived limit functors in turn involves constructing a "correct" (metrizable) topology on the order complex P|P| of a partially ordered metrizable space PP (such as the hyperspace K(X)K(X) of nonempty compact subsets of XX with the Hausdorff metric). It turns out that three natural approaches (by using the space of measurable functions, the space of probability measures, or the usual embedding K(X)C(X;R)K(X)\to C(X;\mathbb R)) all lead to the same topology on P|P|.

Keywords

Cite

@article{arxiv.1809.00022,
  title  = {Lim colim versus colim lim. II: Derived limits over a pospace},
  author = {Sergey A. Melikhov},
  journal= {arXiv preprint arXiv:1809.00022},
  year   = {2022}
}

Comments

29 pages. v2: Minor changes (Proposition 3.5 from v1 has migrated to arXiv:1106.3249, where it is now called Proposition 26.14)

R2 v1 2026-06-23T03:51:04.605Z