The telescope approach to embeddability of compacta

Abstract

We show that an n-dimensional compactum X embeds in R^m, where m>3(n+1)/2, if and only if X x X - Δadmits an equivariant map to S^{m-1}. In particular, X embeds in R^{2n}, n>3, iff the top power of the (twisted) Euler class of the factor-exchanging involution on X x X - Δis trivial. Assuming that X quasi-embeds in R^{2n} (i.e. is an inverse limit of n-polyhedra, embeddable in R^{2n}), this is equivalent to the vanishing of an obstruction in lim^1 H^{2n-1}(K_i) over compact subsets K_i\subset X x X - Δ. One application is that an n-dimensional ANR embeds in R^{2n} if it quasi-embeds in R^{2n-1}, n>3. We construct an ANR of dimension n>1, quasi-embeddable but not embeddable in R^{2n}, and an AR of dimension n>1, which does not "movably" embed in R^{2n}. These examples come close to, but don't quite resolve, Borsuk's problem: does every n-dimensional AR embed in R^{2n}? In the affirmative direction, we show that an n-dimensional compactum X embeds in R^{2n}, n>3, if H^n(X)=0 and H^{n+1}(X,X-pt)=0 for every pt\in X. There are applications in the entire metastable range as well. An n-dimensional compactum X with H^{n-i}(X-pt)=0 for each pt\in X and all i\le k embeds in R^{2n-k}. This generalizes Bryant and Mio's result that k-connected n-dimensional generalized manifolds embed in R^{2n-k}. Also, an acyclic compactum X embeds in R^m iff X x I embeds in R^{m+1} iff X x (triod) embeds in R^{m+2}. As a byproduct, we answer a question of T. Banakh on stable embeddability of the Menger cube.