Topological properties of Eschenburg spaces and 3-Sasakian manifolds

The authors examine topological properties of the 7-dimensional Eschenburg biquotients diag(z^k1,z^k2,z^k3)\SU(3)/diag(z^l1,z^l2,z^l3). A subfamily of these spaces carry a 3-Sasakian metric. The authors show that among this subfamily there exist many 3-Sasakian spaces which are homeomorphic but not diffeomorphic. In addition, they construct a pair of 3-Sasakian spaces which are diffeomorphic, thus giving the first example of a manifold which carries two non-isometric 3-Sasakian metrics. This answers an open question of C. Boyer and K.Galicki. Among the general family, the authors construct many pairs of positively curved Eschenburg spaces which are homeomorphic but not diffeomorphic. Such pairs were first constructed by Kreck-Stolz among the special subfamily of Aloff-Wallach spaces.