Lemma 42.40.1. Let $(S, \delta )$ be as in Situation 42.7.1. Let $X$ be locally of finite type over $S$. Let $\mathcal{E}$, $\mathcal{F}$ be finite locally free sheaves on $X$ of ranks $r$, $r - 1$ which fit into a short exact sequence

\[ 0 \to \mathcal{O}_ X \to \mathcal{E} \to \mathcal{F} \to 0 \]

Then we have

\[ c_ r(\mathcal{E}) = 0, \quad c_ j(\mathcal{E}) = c_ j(\mathcal{F}), \quad j = 0, \ldots , r - 1 \]

in $A^*(X)$.

**Proof.**
By Lemma 42.35.3 it suffices to show that if $X$ is integral then $c_ j(\mathcal{E}) \cap [X] = c_ j(\mathcal{F}) \cap [X]$. Let $(\pi : P \to X, \mathcal{O}_ P(1))$, resp. $(\pi ' : P' \to X, \mathcal{O}_{P'}(1))$ denote the projective space bundle associated to $\mathcal{E}$, resp. $\mathcal{F}$. The surjection $\mathcal{E} \to \mathcal{F}$ gives rise to a closed immersion

\[ i : P' \longrightarrow P \]

over $X$. Moreover, the element $1 \in \Gamma (X, \mathcal{O}_ X) \subset \Gamma (X, \mathcal{E})$ gives rise to a global section $s \in \Gamma (P, \mathcal{O}_ P(1))$ whose zero set is exactly $P'$. Hence $P'$ is an effective Cartier divisor on $P$ such that $\mathcal{O}_ P(P') \cong \mathcal{O}_ P(1)$. Hence we see that

\[ c_1(\mathcal{O}_ P(1)) \cap \pi ^*\alpha = i_*((\pi ')^*\alpha ) \]

for any cycle class $\alpha $ on $X$ by Lemma 42.31.1. By Lemma 42.38.2 we see that $\alpha _ j = c_ j(\mathcal{F}) \cap [X]$, $j = 0, \ldots , r - 1$ satisfy

\[ \sum \nolimits _{j = 0}^{r - 1} (-1)^ jc_1(\mathcal{O}_{P'}(1))^ j \cap (\pi ')^*\alpha _ j = 0 \]

Pushing this to $P$ and using the remark above as well as Lemma 42.26.4 we get

\[ \sum \nolimits _{j = 0}^{r - 1} (-1)^ j c_1(\mathcal{O}_ P(1))^{j + 1} \cap \pi ^*\alpha _ j = 0 \]

By the uniqueness of Lemma 42.38.2 we conclude that $c_ r(\mathcal{E}) \cap [X] = 0$ and $c_ j(\mathcal{E}) \cap [X] = \alpha _ j = c_ j(\mathcal{F}) \cap [X]$ for $j = 0, \ldots , r - 1$. Hence the lemma holds.
$\square$

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