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Is the next statement true?

Let $M$ be a non-compact linearly connected oriented topological manifold of dimension $n$, and let $M^+$ be the one-point compactification of $M$. Then there is a canonical isomorphism $$ H^q(M;\mathbb{Z})\cong\widetilde{H}_{n-q}(M^+;\mathbb{Z}) $$ where $0\leqslant q\leqslant n$ and $\widetilde{H}$ means the reduced homology.

If this is true, where could I find a proof?

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    $\begingroup$ @IgorBelegradek OP specifies that M should be orientable (otherwise probably the LHS should be taken with $w_1$-twisted coefficients). $\endgroup$
    – mme
    Commented Jan 3, 2023 at 19:49
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    $\begingroup$ It fails for $M = S^1 \setminus C$ where $C$ is a Cantor set in $S^1$. $\endgroup$ Commented Jan 3, 2023 at 20:47
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    $\begingroup$ I suppose it fails for essentially the same reason when $M= \mathbb{Z}$. $\endgroup$ Commented Jan 3, 2023 at 22:51
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    $\begingroup$ Or even a sphere minus more than one point. $\endgroup$
    – Jim Conant
    Commented Jan 4, 2023 at 2:55
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    $\begingroup$ For a sphere minus finitely many points it looks ok, doesn't it? The version with homology and cohomology swapped (i.e. $H_q(M) \cong \widetilde{H}^{n-q}(M^+)$) is usual Poincaré duality combined with the observation that, if the extra point in the $1$-point compactification has a contractible neighbourhood basis, cohomology with compact support agrees with this reduced cohomology. And for $M$ finite type, there shouldn't be any problems dualizing this. $\endgroup$ Commented Jan 4, 2023 at 8:01

1 Answer 1

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As pointed out in the comments, this sometimes fails if $M$ is not the homotopy type of a compact manifold with boundary, but is true if $M \cong \mathrm{int}(\bar{M})$ for a compact, orientable $n$-manifold $\bar{M}$. Here is a short argument which derives both classical Poincare duality and this version at the same time.

Recall that for a finite space $X$, the Spanier-Whitehead dual $X_+^\vee$ is given by the function spectrum $F(\Sigma^\infty_+ X,S^0)$. This spectrum has the property that $H_*(X)\cong H^{-*}(X_+^\vee)$ and $H^*(X) \cong H_{-*}(X_+ ^\vee)$. Note the right hand sides are "reduced" because they are the (co)homology of a spectrum

Notice that $M^+ \cong \bar{M}/\partial \bar{M}$, so it suffices to show $H^q(\bar{M}) \cong \bar{H}_{n-q}(\bar{M}/\partial \bar{M})$. Let $i:\bar{M} \rightarrow \mathbb{R}^N$ be a codimension $c$ embedding with a disk bundle neighborhood $\nu$. Dolde-Puppe proved that in these circumstances $\Sigma^{n+c}M_+^\vee \simeq \Sigma^\infty \bar{M}^{\nu}/\partial\bar{M}^{\nu|_{\partial M}}$, where $\bar{M}^\nu$ denotes the Thom complex of $\nu$. By (relative) Thom isomorphism, the latter has the (co)homology of $\Sigma^c (\bar{M}/\partial \bar{M})$.

One can now apply (co)homology to this equivalence, and then apply the (co)homological Thom isomorphism. Then we get two statements of Poincare duality:

$H^{n+c-*}(\bar{M})\cong \bar{H}_{*-c}(\bar{M}/\partial\bar{M})$

$H_{n+c-*}(\bar{M})\cong \bar{H}^{*-c}(\bar{M}/\partial\bar{M})$

Letting $n+c-*=q$, the first reduces to $H^q(\bar{M})\cong \bar{H}_{n-q}(\bar{M}/\partial \bar{M}))$ and the second reduces to classical Poincare duality, as desired. A variation of this argument should work in the slightly more general case where $M$ is homotopy equivalent to a finite complex, but not necessarily the interior of a closed manifold.

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    $\begingroup$ Less fancy, classical Lefschetz duality gives $H^q(M)\cong H^q(\overline{M})\cong H_{n-q}(\overline{M},\partial \overline{M})\cong \widetilde{H}_{n-q}(\overline{M}/\partial \overline{M})\cong \widetilde{H}_{n-q}(M^+)$. $\endgroup$ Commented Jan 4, 2023 at 20:41
  • $\begingroup$ @archipelago Totally, my initial thought at this question was that it was answered by Lefschetz duality. My guess is the question was asked because it is more commonly stated $H_q(M) \cong \bar{H}^{n-q}(M^+)$, so I was hoping to give an indication as to why it can be stated either way. $\endgroup$ Commented Jan 4, 2023 at 20:46

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