Splitting vector bundles and A1-fundamental groups of higher-dimensional varieties

摘要

We study aspects of the A~1-homotopy classification problem in dimensions at least 3 and, to this end, we investigate the problem of computing A~1-homotopy groups of some A~1-connected smooth varieties of dimension at least 3. Using these computations, we construct pairs of A~1-connected smooth proper varieties all of whose A~1-homotopy groups are abstractly isomorphic, yet which are not A~1-weakly equivalent. The examples come from pairs of Zariski locally trivial projective space bundles over projective spaces and are of the smallest possible dimension. Projectivizations of vector bundles give rise to A~1-fiber sequences, and when the base of the fibration is an A~1-connected smooth variety, the associated long exact sequence of A~1-homotopy groups can be analyzed in detail. In the case of the projectivization of a rank 2 vector bundle, the structure of the A~1-fundamental group depends on the splitting behavior of the vector bundle via a certain obstruction class. For projective bundles of vector bundles of rank at least 3, the A~1-fundamental group is insensitive to the splitting behavior of the vector bundle, but the structure of higher A~1-homotopy groups is influenced by an appropriately defined higher obstruction class.