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Let $X$ be a finite CW-complex of $n$.

  1. For $i\geq 2$, $\pi_i (X)$ is a $\mathbb{Z}\pi_1 (X)$-module.

  2. for $i\geq 2$, $H_i (\tilde{X})$ is a finitely generated $\mathbb{Z}\pi_1 (X)$-module, where $\tilde{X}$ is the universal covering of $X$.

I know that the connection between homotopy groups and homology groups is given by the Hurewicz-Theorem. But my question is that:

My question: Can we express $\pi_i (X)$ (as a $\mathbb{Z}\pi_1 (X)$-module) in terms of $H_i (X)$'s (as $\mathbb{Z}$-module) or $H_i (\tilde{X})$ (as $\mathbb{Z}\pi_1 (X)$-module) for $2\leq i\leq n$? (particularly when they are free).

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  • $\begingroup$ What makes you believe that such a relation could exist? $\endgroup$ Commented May 9, 2023 at 5:09
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    $\begingroup$ The homology groups of $S^2$ are all free while it has infinitely many non free homotopy groups. $\endgroup$ Commented May 9, 2023 at 5:20
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    $\begingroup$ @M.Ramana you're not, $S^2$ is simply connected. $\endgroup$ Commented May 9, 2023 at 8:03
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    $\begingroup$ @IgorKhavkine that’s not the Pistnikov system. Also, there’s no way to recover a homotopy type from the homotopy groups equipped with the action of the fundamental groups. If that were true, simply connected spaces would be determined by their homotopy groups. $\endgroup$ Commented May 10, 2023 at 4:20
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    $\begingroup$ @IgorKhavkine homotopy types can be captured by algebraic data, that's correct, in several ways actually, but homotopy groups don't suffice in any single way, not even when regarded as modules over the fundamental group. Postnikov systems are more complex. $\endgroup$ Commented May 10, 2023 at 8:45

1 Answer 1

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Not sure if you're already aware of an exact sequence which implies Hurewicz's theorem, but if $X$ is a space with $\pi_i(X)=0$ for $1<i<n$ then we have $$H_{n+1}(X)\to H_{n+1}(\pi)\to \pi_n(X)_{\pi}\to H_n(X)\stackrel{\psi}{\to} H_n(\pi)\to0$$ where $\pi=\pi_1(X)$ is the fundamental group and $H_*(\pi)$ is discrete group cohomology, and $\pi_n(X)_{\pi}$ denotes the coinvariants. See e.g. Ken Brown "Cohomology of Groups" exercise VII.7.6.

Generally, without assumptions on the homotopy groups, that map $\psi$ exists (in all degrees) and is canonical: You take a projective resolution of $\mathbb Z$ over $\mathbb Z\pi$, and you take the complex of free $\mathbb Z\pi$-modules $C_*(\text{universal cover of }X)$, to build a canonical chain map between those resolutions that induces $\psi$. This is also explained in the above reference somewhere in Chapter II.

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  • $\begingroup$ I didn't know that. That's a good point. Thank you very much for your help. $\endgroup$
    – M.Ramana
    Commented May 9, 2023 at 5:30
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    $\begingroup$ @Chris Gerig: You are assuming $\pi _i(X)=0$ for $1<i<n$, right? $\endgroup$
    – abx
    Commented May 9, 2023 at 6:08

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