Universal coefficient theorem for group homology

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For coefficients in an abelian group

Suppose G is a group and M is an abelian group. The universal coefficients theorem for group homology describes the homology groups for trivial group action of G on M in terms of the homology groups for trivial group action of G on \mathbb{Z}.

Explicitly, it states that there is a natural short exact sequence of abelian groups:

0 \to H_p(G;\mathbb{Z}) \otimes M \to H_p(G;M) \to \operatorname{Tor}(H_{p-1}(G;\mathbb{Z}),M) \to 0

The sequence splits (though not naturally) to give that:

H_p(G;M) \cong (H_p(G;\mathbb{Z}) \otimes M) \oplus \operatorname{Tor}(H_{p-1}(G;\mathbb{Z}),M)

Typical case of finitely generated abelian groups

Suppose H_p(G;\mathbb{Z}) \cong \mathbb{Z}^{r_p} \oplus T_p for some finite group T_p and H_{p-1}(G;\mathbb{Z}) \cong \mathbb{Z}^{r_{p-1}} \oplus T_{p-1} for some finite group T_{p-1}.

Suppose further that:

T_p \cong \mathbb{Z}/a_1\mathbb{Z} \oplus \mathbb{Z}/a_2\mathbb{Z} \oplus \dots \oplus \mathbb{Z}/a_s\mathbb{Z}


T_{p-1} \cong \mathbb{Z}/b_1\mathbb{Z} \oplus \mathbb{Z}/b_2\mathbb{Z} \oplus \dots \oplus \mathbb{Z}/b_t\mathbb{Z}

Then we have:

H_p(G;M) \cong M^{r_p} \oplus (T_p \otimes M) \oplus \operatorname{Tor}(T_{p-1},M)

where we have:

T_p \otimes M \cong \bigoplus_{1 \le i \le s} M/a_iM


\operatorname{Tor}(T_{p-1},M) \cong \bigoplus_{1 \le i \le t} \operatorname{Ann}_M(b_i)

where \operatorname{Ann}_M(b_i) = \{ x \in M \mid b_ix = 0 \}

Thus, overall:

H_p(G;M) \cong M^{r_p} \oplus \bigoplus_{1 \le i \le s} M/a_iM \oplus \bigoplus_{1 \le i \le t} \operatorname{Ann}_M(b_i)

If, further, M is a finitely generated abelian group, of the form:

M \cong \mathbb{Z}^w \oplus \mathbb{Z}/c_1\mathbb{Z} \oplus \mathbb{Z}/c_2\mathbb{Z} \oplus \dots \oplus \mathbb{Z}/c_u\mathbb{Z}

Then the expressions simpliy further:

T_p \otimes M \cong T_p^w \oplus \sum_{1 \le i \le s, 1 \le j \le u} \mathbb{Z}/\operatorname{gcd}(a_i,c_j)\mathbb{Z}


\operatorname{Tor}(T_{p-1},M) \cong \bigoplus_{1 \le i \le t, 1 \le j \le u} \mathbb{Z}/\operatorname{gcd}(b_i,c_j)\mathbb{Z}

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