Let $M$ be smooth oriented manifold where $M=X\times F$, $X$ and $F$ smooth oriented manifolds. We note by $[M]$ the fundamental class of $M$. Is this equality true: $$[X\times F] = [X]\times [F]?$$
2026-03-29 04:26:30.1774758390
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Fundamental class of products of spaces
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This statement is proved in Propositions 80.10 and Proposition 80.11 of my lecture notes: https://www.uni-regensburg.de/Fakultaeten/nat_Fak_I/friedl/papers/1a-uptodate.pdf Getting the signs right was painful.
The Künneth theorem with real coefficients gives an isomorphism $$H_*(X\times F)\cong H_*(X)\otimes H_*(F).$$ Under this isomorphism, $\left[X\times F\right]$ corresponds to $\left[X\right]\otimes \left[F\right]$.