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For $n,m\geq 1$ the spaces $X=S^n\times S^m$ and $Y=S^n \vee S^m \vee S^{n+m}$ have isomorphic homology and cohomology groups. The cohomology groups are given by $H^i(X)=H^i(Y)=\mathbb{Z}$ for $i=0,n,m,n+m$ and zero else. The spaces $X$ and $Y$ are not homotopy equivalent however, since their cohomology rings are not isomorphic. For $X$, the cup product of a generator of the $n$-th cohomology with a generator of the $m$-th cohomology yields a generator of the $(m+n)$-th cohomology. The space $Y$ has trivial cup products in positive degree.

Using the suspension isomorphism we find that the unreduced suspensions $S(X)$ and $S(Y)$ have isomorphic cohomology and homology. The cup product on a suspension is trivial, so the cohomology rings of $S(X)$ and $S(Y)$ are isomorphic too. In other words, (co)homology does not seem to be able to distinguish these spaces. Are $S(X)$ and $S(Y)$ homotopy equivalent?

Margaret
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    Yes, they are homotopy equivalent. See https://math.stackexchange.com/questions/301402/suspension-of-a-product-tricky-homotopy-equivalence – Charlie Sep 29 '23 at 08:01
  • @Charlie Doesn't this only show $S(S^m\times S^n)\simeq S(S^n) \vee S(S^m)\vee S(S^m \land S^n)\simeq S^{n+1}\vee S^{m+1}\vee S^{m+n+1}$? Why is the right hand side homotopy equivalent to $S(S^n \vee S^m \vee S^{n+m})$? – Margaret Sep 29 '23 at 08:34
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    Smash product distribute over finite wedge sums: https://math.stackexchange.com/questions/2900802/smash-products-distributes-over-wedge – Charlie Sep 29 '23 at 08:52
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    I see. What I was missing is that $\Sigma X \simeq S^1 \land X$. – Margaret Sep 29 '23 at 09:08

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Answer from the comments. Thanks to @Charlie for helping me out.

Recall the following facts (for topological spaces $X,Y,Z$):

  • $\Sigma X \simeq S^1 \land X,$
  • $(X \vee Y)\wedge Z \simeq (X \wedge Z) \vee (Y \wedge Z)$ as proved here,
  • $\Sigma (X\times Y) \simeq \Sigma X \lor \Sigma Y \lor \Sigma (X\land Y)$ as proved here,
  • If $X$ is a CW-complex, then $\Sigma X\simeq SX$ as proved here,
  • $S^n \land S^m \simeq S^{n+m}.$

Therefore

$$S(S^n\times S^m)\simeq \Sigma(S^n\times S^m)\simeq \Sigma S^n \vee \Sigma S^m \vee \Sigma (S^n \land S^m)\simeq (S^1 \land S^n) \vee (S^1 \land S^m) \vee (S^1 \land S^{n+m}) \simeq S^1\land (S^n \vee S^m \vee S^{n+m})\simeq \Sigma (S^n \vee S^m \vee S^{n+m})\simeq S(S^n \vee S^m \vee S^{n+m}).$$

Margaret
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