Definable (co)homology, pro-torus rigidity, and (co)homological classification

Abstract

We show that the classical homology theory of Steenrod may be enriched with descriptive set-theoretic information. We prove that the resulting definable homology theory provides a strictly finer invariant than Steenrod homology for compact metrizable spaces up to homotopy. In particular, we show that pro-tori are completely classified up to homeomorphism by their definable homology. This is in contrast with the fact that, for example, there exist uncountably many pairwise non-homeomorphic solenoids with the same Steenrod homology groups. We similarly develop a definable cohomology theory which strengthens \v{C}ech cohomology and we show that it completely classifies complements of pro-tori up to homeomorphism. We also apply definable cohomology theory to the study of the space $\left[ X,S^{2}\right] $ of homotopy classes of continuous functions from a solenoid complement $X$ to the $2$-sphere, which was initiated by Borsuk and Eilenberg in 1936. It was proved by Eilenberg and Steenrod in 1940 that the space $\left[ X,S^{2}\right] $ is uncountable. We will strengthen this result, by showing that each orbit of the canonical action $\mathrm{Homeo}% \left( X\right) \curvearrowright \left[ X,S^{2}\right] $ is countable, and hence that such an action has uncountably many orbits. This can be seen as a rigidity result, and will be deduced from a rigidity result for definable automorphisms of the \v{C}ech cohomology of $X$. We will also show that these results still hold if one replaces solenoids with pro-tori. We conclude by applying the machinery developed herein to bound the Borel complexity of several well-studied classification problems in mathematics, such as that of automorphisms of continuous-trace $C^{*}$-algebras up to unitary equivalence, or that of Hermitian line bundles, up to isomorphism, over a locally compact second countable space.