Smooth varieties up to A~1-homotopy and algebraic h-cobordisms

摘要

We start to study the problem of classifying smooth proper varieties over a field k from the standpoint of A~1-homotopy theory. Motivated by the topological theory of surgery, we discuss the problem of classifying up to isomorphism all smooth proper varieties having a specified A~1-homotopy type. Arithmetic considerations involving the sheaf of A~1-connected components lead us to introduce several different notions of connectedness in A~1-homotopy theory. We provide concrete links between these notions, connectedness of points by chains of affine lines, and various rationality properties of algebraic varieties (e.g., rational connectedness).We introduce the notion of an A~1-h-cobordism, an algebro-geometric analog of the topological notion of h-cobordism, and use it as a tool to produce non-trivial A~1-weak equivalences of smooth proper varieties. Also, we give explicit computations of refined A~1-homotopy invariants, such as the A~1-fundamental sheaf of groups, for some A~1-connected varieties. We observe that the A~1-fundamental sheaf of groups plays a central yet mysterious role in the structure of A~1-h-cobordisms. As a consequence of these observations, we completely solve the classification problem for rational smooth proper surfaces over an algebraically closed field: while there exist arbitrary dimensional moduli of such surfaces, there are only countably many A~1-homotopy types, each uniquely determined by the isomorphism class of its A~1-fundamental sheaf of groups.