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If $X^\bullet$ and $Y^\bullet$ are chain complexes over a field, we know from Kunneth theorem that $$H^*((X\otimes Y)^\bullet)\cong H^*(X^\bullet)\otimes H^*(Y^\bullet) $$

I want to know if there is a similar theorem for internal homs: for chain complexes $X^\bullet$ and $Y^\bullet$, we set

$$[X,Y]^\bullet = \underset{k\in\mathbb Z}{\prod} [X^k,Y^{k+\bullet}]$$

Do we have ?

$$ H^*([X,Y]^\bullet)=[H^*(X^\bullet),H^*(Y^\bullet)]\qquad \mbox{or} \qquad H^n([X,Y])=\underset{j-i=n}{\prod}[H^i(X),H^j(Y)]$$

If this isn't true, is there some kind of relation between the two sides, in general or in some special cases such as with bounded complexes etc?

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  • $\begingroup$ Over a field, every chain complex is quasi-isomorphic to its homology, viewed as a chain complex with zero differential. It means that every chain complex is quasi-isomorphic to both the direct sum and the direct product of complexes concentrated in a single degree. The statement that you want seems to follow easily. $\endgroup$ Commented Oct 7, 2021 at 11:38
  • $\begingroup$ Thank you so much. I am just very worried about the various signs and whether I am mixing up direct products with direct sums. It would be awesome if there is some reference. $\endgroup$
    – Kunneth
    Commented Oct 7, 2021 at 11:42

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