The Stacks project

50.4 Cup product

Consider the maps $\Omega ^ p_{X/S} \times \Omega ^ q_{X/S} \to \Omega ^{p + q}_{X/S}$ given by $(\omega , \eta ) \longmapsto \omega \wedge \eta $. Using the formula for $\text{d}$ given in Section 50.2 and the Leibniz rule for $\text{d} : \mathcal{O}_ X \to \Omega _{X/S}$ we see that $\text{d}(\omega \wedge \eta ) = \text{d}(\omega ) \wedge \eta + (-1)^{\deg (\omega )} \omega \wedge \text{d}(\eta )$. This means that $\wedge $ defines a morphism

50.4.0.1
\begin{equation} \label{derham-equation-wedge} \wedge : \text{Tot}( \Omega ^\bullet _{X/S} \otimes _{p^{-1}\mathcal{O}_ S} \Omega ^\bullet _{X/S}) \longrightarrow \Omega ^\bullet _{X/S} \end{equation}

of complexes of $p^{-1}\mathcal{O}_ S$-modules.

Combining the cup product of Cohomology, Section 20.31 with (50.4.0.1) we find a $H^0(S, \mathcal{O}_ S)$-bilinear cup product map

\[ \cup : H^ i_{dR}(X/S) \times H^ j_{dR}(X/S) \longrightarrow H^{i + j}_{dR}(X/S) \]

For example, if $\omega \in \Gamma (X, \Omega ^ i_{X/S})$ and $\eta \in \Gamma (X, \Omega ^ j_{X/S})$ are closed, then the cup product of the de Rham cohomology classes of $\omega $ and $\eta $ is the de Rham cohomology class of $\omega \wedge \eta $, see discussion in Cohomology, Section 20.31.

Given a commutative diagram

\[ \xymatrix{ X' \ar[r]_ f \ar[d] & X \ar[d] \\ S' \ar[r] & S } \]

of schemes, the pullback maps $f^* : R\Gamma (X, \Omega ^\bullet _{X/S}) \to R\Gamma (X', \Omega ^\bullet _{X'/S'})$ and $f^* : H^ i_{dR}(X/S) \longrightarrow H^ i_{dR}(X'/S')$ are compatible with the cup product defined above.

Lemma 50.4.1. Let $p : X \to S$ be a morphism of schemes. The cup product on $H^*_{dR}(X/S)$ is associative and graded commutative.

Proof. This follows from Cohomology, Lemmas 20.31.5 and 20.31.6 and the fact that $\wedge $ is associative and graded commutative. $\square$

Remark 50.4.2. Let $p : X \to S$ be a morphism of schemes. Then we can think of $\Omega ^\bullet _{X/S}$ as a sheaf of differential graded $p^{-1}\mathcal{O}_ S$-algebras, see Differential Graded Sheaves, Definition 24.12.1. In particular, the discussion in Differential Graded Sheaves, Section 24.32 applies. For example, this means that for any commutative diagram

\[ \xymatrix{ X \ar[d]_ p \ar[r]_ f & Y \ar[d]^ q \\ S \ar[r]^ h & T } \]

of schemes there is a canonical relative cup product

\[ \mu : Rf_*\Omega ^\bullet _{X/S} \otimes _{q^{-1}\mathcal{O}_ T}^\mathbf {L} Rf_*\Omega ^\bullet _{X/S} \longrightarrow Rf_*\Omega ^\bullet _{X/S} \]

in $D(Y, q^{-1}\mathcal{O}_ T)$ which is associative and which on cohomology reproduces the cup product discussed above.

Remark 50.4.3. Let $f : X \to S$ be a morphism of schemes. Let $\xi \in H_{dR}^ n(X/S)$. According to the discussion Differential Graded Sheaves, Section 24.32 there exists a canonical morphism

\[ \xi ' : \Omega ^\bullet _{X/S} \to \Omega ^\bullet _{X/S}[n] \]

in $D(f^{-1}\mathcal{O}_ S)$ uniquely characterized by (1) and (2) of the following list of properties:

  1. $\xi '$ can be lifted to a map in the derived category of right differential graded $\Omega ^\bullet _{X/S}$-modules, and

  2. $\xi '(1) = \xi $ in $H^0(X, \Omega ^\bullet _{X/S}[n]) = H^ n_{dR}(X/S)$,

  3. the map $\xi '$ sends $\eta \in H^ m_{dR}(X/S)$ to $\xi \cup \eta $ in $H^{n + m}_{dR}(X/S)$,

  4. the construction of $\xi '$ commutes with restrictions to opens: for $U \subset X$ open the restriction $\xi '|_ U$ is the map corresponding to the image $\xi |_ U \in H^ n_{dR}(U/S)$,

  5. for any diagram as in Remark 50.4.2 we obtain a commutative diagram

    \[ \xymatrix{ Rf_*\Omega ^\bullet _{X/S} \otimes _{q^{-1}\mathcal{O}_ T}^\mathbf {L} Rf_*\Omega ^\bullet _{X/S} \ar[d]_{\xi ' \otimes \text{id}} \ar[r]_-\mu & Rf_*\Omega ^\bullet _{X/S} \ar[d]^{\xi '} \\ Rf_*\Omega ^\bullet _{X/S}[n] \otimes _{q^{-1}\mathcal{O}_ T}^\mathbf {L} Rf_*\Omega ^\bullet _{X/S} \ar[r]^-\mu & Rf_*\Omega ^\bullet _{X/S}[n] } \]

    in $D(Y, q^{-1}\mathcal{O}_ T)$.


Comments (0)


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.




In order to prevent bots from posting comments, we would like you to prove that you are human. You can do this by filling in the name of the current tag in the following input field. As a reminder, this is tag 0FM1. Beware of the difference between the letter 'O' and the digit '0'.