Wiedersehen metrics and exotic involutions of Euclidean spheres
  We provide explicit, simple, geometric formulas for free involutions rho of Euclidean spheres that are not conjugate to the antipodal involution. Therefore the quotient S^n/rho is a manifold that is homotopically equivalent but not diffeomorphic to RP^n. We use these formulas for constructing explicit non-trivial elements in pi_1 Diff(S^5) and pi_1 Diff(S^13) and to provide explicit formulas for non-cancellation phenomena in group actions. 
Uwe Abresch (Ruhr-Universitaet Bochum), Carlos Duran (Unicamp), Thomas   Puettmann (Ruhr-Universitaet Bochum), A. Rigas (Uni...