STABLE DIFFEOMORPHISM GROUPS OF 4-MANIFOLDS

A localisation of the category of n-manifolds is introduced by formally inverting the connected sum construction with a chosen n-manifold Y. On the level of automorphism groups, this leads to the stable diffeomorphism groups of n-manifolds. In dimensions 0 and 2, this is connected to the stable homotopy groups of spheres and the stable mapping class groups of Riemann surfaces. In dimension 4 there are many essentially different candidates for the n-manifold Y to choose from. It is shown that the Bauer-Furuta invariants provide invariants in the case Y = (CP) over bar (2), which is related to the birational classification of complex surfaces. This will be the case for other Y only after localisation of the target category. In this context, it is shown that the K 3-stable Bauer-Fluruta invariants determine the S-2 x S-2-stable invariants.