The Stacks project

Lemma 81.11.1. In Situation 81.2.1 let

\[ \xymatrix{ X' \ar[r]_{g'} \ar[d]_{f'} & X \ar[d]^ f \\ Y' \ar[r]^ g & Y } \]

be a fibre product diagram of good algebraic spaces over $B$. Assume $f : X \to Y$ proper and $g : Y' \to Y$ flat of relative dimension $r$. Then also $f'$ is proper and $g'$ is flat of relative dimension $r$. For any $k$-cycle $\alpha $ on $X$ we have

\[ g^*f_*\alpha = f'_*(g')^*\alpha \]

in $Z_{k + r}(Y')$.

Proof. The assertion that $f'$ is proper follows from Morphisms of Spaces, Lemma 66.40.3. The assertion that $g'$ is flat of relative dimension $r$ follows from Morphisms of Spaces, Lemmas 66.34.3 and 66.30.4. It suffices to prove the equality of cycles when $\alpha = [W]$ for some integral closed subspace $W \subset X$ of $\delta $-dimension $k$. Note that in this case we have $\alpha = [\mathcal{O}_ W]_ k$, see Lemma 81.6.3. By Lemmas 81.8.3 and 81.10.5 it therefore suffices to show that $f'_*(g')^*\mathcal{O}_ W$ is isomorphic to $g^*f_*\mathcal{O}_ W$. This follows from cohomology and base change, see Cohomology of Spaces, Lemma 68.11.2. $\square$


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