Lemma 42.38.4. Let $(S, \delta )$ be as in Situation 42.7.1. Let $X$, $Y$ be locally of finite type over $S$. Let $\mathcal{E}$ be a finite locally free sheaf of rank $r$ on $X$. Let $p : X \to Y$ be a proper morphism. Let $\alpha $ be a $k$-cycle on $X$. Let $\mathcal{E}$ be a finite locally free sheaf on $Y$. Then

**Proof.**
Let $(\pi : P \to Y, \mathcal{O}_ P(1))$ be the projective bundle associated to $\mathcal{E}$. Then $P_ X = X \times _ Y P$ is the projective bundle associated to $p^*\mathcal{E}$ and $\mathcal{O}_{P_ X}(1)$ is the pullback of $\mathcal{O}_ P(1)$. Write $\alpha _ j = c_ j(p^*\mathcal{E}) \cap \alpha $, so $\alpha _0 = \alpha $. By Lemma 42.38.2 we have

in the chow group of $P_ X$. Consider the fibre product diagram

Apply proper pushforward $p'_*$ (Lemma 42.20.3) to the displayed equality above. Using Lemmas 42.26.4 and 42.15.1 we obtain

in the chow group of $P$. By the characterization of Lemma 42.38.2 we conclude. $\square$

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