Affine models for Noetherian schemes

Abstract

Let $S$ be a base scheme, assumed separated and Noetherian. We define \emph{adequate classes} of morphisms of $S$-schemes by formalizing certain properties of homotopy equivalences of complex algebraic varieties. Other examples of adequate classes include morphisms of varieties over an algebraically closed field of positive characteristic which induce an isomorphism of étale cohomology, and weak equivalences in the cdh topology. An \emph{affine model} for a Noetherian scheme $X$ over $S$ is an affine scheme $M_X$ equipped with an adequate morphism $M_X\to X$. In this paper we construct affine models for arbitrary separated schemes of finite type over $S$. Our construction can be viewed as a generalization of the Jouanolou trick. As an application, we construct a mixed Hodge structure on the Leray spectral sequence of an arbitrary proper morphism $f:X\to Y$ of complex algebraic varieties, generalizing an argument by Donu Arapura which assumed $Y$ quasi-projective and $f$ projective.