Lemma 50.13.1. Let f : X \to Y be a smooth proper morphism of schemes. Let N and n_1, \ldots , n_ N \geq 0 be integers and let \xi _ i \in H^{n_ i}_{dR}(X/Y), 1 \leq i \leq N. Assume for all points y \in Y the images of \xi _1, \ldots , \xi _ N in H^*_{dR}(X_ y/y) form a basis over \kappa (y). Then the map
\bigoplus \nolimits _{i = 1}^ N \mathcal{O}_ Y[-n_ i] \longrightarrow Rf_*\Omega ^\bullet _{X/Y}
associated to \xi _1, \ldots , \xi _ N is an isomorphism.
Proof.
By Lemma 50.3.5 Rf_*\Omega ^\bullet _{X/Y} is a perfect object of D(\mathcal{O}_ Y) whose formation commutes with arbitrary base change. Thus the map of the lemma is a map a : K \to L between perfect objects of D(\mathcal{O}_ Y) whose derived restriction to any point is an isomorphism by our assumption on fibres. Then the cone C on a is a perfect object of D(\mathcal{O}_ Y) (Cohomology, Lemma 20.49.7) whose derived restriction to any point is zero. It follows that C is zero by More on Algebra, Lemma 15.75.7 and a is an isomorphism. (This also uses Derived Categories of Schemes, Lemmas 36.3.5 and 36.10.7 to translate into algebra.)
\square
Comments (0)