The Stacks project

Definition 81.10.1. In Situation 81.2.1 let $X, Y/B$ be good. Let $f : X \to Y$ be a morphism over $B$. Assume $f$ is flat of relative dimension $r$.

  1. Let $Z \subset Y$ be an integral closed subspace of $\delta $-dimension $k$. We define $f^*[Z]$ to be the $(k+r)$-cycle on $X$ associated to the scheme theoretic inverse image

    \[ f^*[Z] = [f^{-1}(Z)]_{k+r}. \]

    This makes sense since $\dim _\delta (f^{-1}(Z)) = k + r$ by Lemma 81.9.1.

  2. Let $\alpha = \sum n_ i [Z_ i]$ be a $k$-cycle on $Y$. The flat pullback of $\alpha $ by $f$ is the sum

    \[ f^* \alpha = \sum n_ i f^*[Z_ i] \]

    where each $f^*[Z_ i]$ is defined as above. The sum is locally finite by Lemma 81.9.2.

  3. We denote $f^* : Z_ k(Y) \to Z_{k + r}(X)$ the map of abelian groups so obtained.

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