Does the Künneth theorem hold for (co-)homology with coefficients in a module

algebraic-topologyhomological-algebra

Let $R$ be a principal ideal domain, and let $M$ be an $R$-module. To compute $H_*(X \times Y;R)$, one typically applies the Künneth theorem, which tells you that there's a split short exact sequence
$$0 \to \bigoplus_{i + j = k} H_i(X; R) \otimes_R H_j(Y;R) \to H_k(X;R) \to \bigoplus_{i + j = k – 1} \operatorname{Tor}^R_1\big(H_i(X;R), H_j(X;R)\big) \to 0\,\text{.}$$
My question is whether this formula still holds true if we're working with homology $H_*(\,\cdot\,,M)$ with coefficients in an $R$-module $M$. (Side question: I'd also be interested in the case where $R$ is no longer is a PID. Is there some general spectral sequence that covers all relevant cases?)

Best Answer

No, the algebraic Kunneth sequence tells us that if we have free chain complexes C,D over the PID R, then we have the short exact sequence:

$0 \rightarrow \bigoplus\limits_{i+j=k} H_i(C) \otimes_R H_j(D) \rightarrow H_k (C \otimes D) \rightarrow \bigoplus\limits_{i+j=k-1} Tor^R_1 (H_i(C),H_j(D)) \rightarrow 0$.

The Eilenberg-Zilber theorem tells us that $S_*(X \times Y,R)$ and $S_*(X,R) \otimes S_*(Y,R)$ are quasi-isomorphic. From these one can deduce the usual Kunneth formula in topology. To figure out the case for coefficients in a module M we can just tensor each of these free chain complexes by $M$ and we preserve the quasi-isomorphism. So we have $S_*(X \times Y,R) \otimes_R M$ is quasi-isomorphic to $S_*(X,R) \otimes S_*(Y,R) \otimes_R M$. Rewriting, the former is $S_*(X \times Y,M)$ and the latter is $S_*(X,R) \otimes_R S_*(Y,M)$.

Applying the algebraic Kunneth sequence tells us that we have a short exact sequence:

$0 \rightarrow \bigoplus\limits_{i+j=k} H_i(X,R) \otimes_R H_j(Y,M) \rightarrow H_k (X \times Y,M) \rightarrow \bigoplus\limits_{i+j=k-1} Tor^R_1 (H_i(X,R),H_j(Y,M)) \rightarrow 0$

And of course you can swap the roles of X and Y if you'd like to pick which space gets which coefficients. The reason why your proposed sequence does not work is because there are modules such that $M \otimes_R M \neq M$.

In the case R is not a PID, you can use the algebraic Kunneth spectral sequence because I believe the Eilenberg-Zilber theorem holds with no conditions on the ring. This spectral sequence involves the higher Tor terms which vanish for PID's.

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