Lemma 50.15.6. Let $X \to S$ be a morphism of schemes. Let $Y \subset X$ be an effective Cartier divisor. Assume the de Rham complex of log poles is defined for $Y \subset X$ over $S$. Let $b \in H^ m_{dR}(X/S)$ be a de Rham cohomology class whose restriction to $Y$ is zero. Then $c_1^{dR}(\mathcal{O}_ X(Y)) \cup b = 0$ in $H^{m + 2}_{dR}(X/S)$.

**Proof.**
This follows immediately from Lemma 50.15.5. Namely, we have

\[ c_1^{dR}(\mathcal{O}_ X(Y)) \cup b = -c_1^{dR}(\mathcal{O}_ X(-Y)) \cup b = -\xi '(b) = -\delta (b|_ Y) = 0 \]

as desired. For the second equality, see Remark 50.4.3. $\square$

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