Lemma 50.23.5. Let $X \to S$ and $i : Z \to X$ be as in Lemma 50.23.3. Given $\alpha \in H^ q(X, \Omega ^ p_{X/S})$ we have $\gamma ^ p(\alpha |_ Z) = i^{-1}\alpha \wedge \gamma ^0(1)$ in $H^ q(Z, \mathcal{H}^ c_ Z(\Omega ^{p + c}_{X/S}))$. Please see proof for notation.

**Proof.**
The restriction $\alpha |_ Z$ is the element of $H^ q(Z, \Omega ^ p_{Z/S})$ given by functoriality for Hodge cohomology. Applying functoriality for cohomology using $\gamma ^ p : \Omega ^ p_{Z/S} \to \mathcal{H}^ c_ Z(\Omega ^{p + c}_{X/S})$ we get get $\gamma ^ p(\alpha |_ Z)$ in $H^ q(Z, \mathcal{H}^ c_ Z(\Omega ^{p + c}_{X/S}))$. This explains the left hand side of the formula.

To explain the right hand side, we first pullback by the map of ringed spaces $i : (Z, i^{-1}\mathcal{O}_ X) \to (X, \mathcal{O}_ X)$ to get the element $i^{-1}\alpha \in H^ q(Z, i^{-1}\Omega ^ p_{X/S})$. Let $\gamma ^0(1) \in H^0(Z, \mathcal{H}_ Z^ c(\Omega ^ c_{X/S}))$ be the image of $1 \in H^0(Z, \mathcal{O}_ Z) = H^0(Z, \Omega ^0_{Z/S})$ by $\gamma ^0$. Using cup product we obtain an element

Using Cohomology, Remark 20.34.9 and wedge product there are canonical maps

By Derived Categories of Schemes, Lemma 36.6.8 the objects $R\mathcal{H}_ Z(\Omega ^ j_{X/S})$ have vanishing cohomology sheaves in degrees $> c$. Hence on cohomology sheaves in degree $c$ we obtain a map

The expression $i^{-1}\alpha \wedge \gamma ^0(1)$ is the image of the cup product $i^{-1}\alpha \cup \gamma ^0(1)$ by the functoriality of cohomology.

Having explained the content of the formula in this manner, by general properties of cup products (Cohomology, Section 20.31), it now suffices to prove that the diagram

is commutative in the category of sheaves on $Z$ (with obvious abuse of notation). This boils down to a simple computation for the maps $\gamma ^ j_{f_1, \ldots , f_ c}$ which we omit; in fact these maps are chosen exactly such that this works and such that $1$ maps to $\frac{\text{d}f_1 \wedge \ldots \wedge \text{d}f_ c}{f_1 \ldots f_ c}$. $\square$

## Post a comment

Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like `$\pi$`

). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.

## Comments (0)