Computability and the growth rate of symplectic homology
  For each n greater than 7 we explicitly construct a sequence of Stein manifolds diffeomorphic to complex affine space of dimension n so that there is no algorithm to tell us in general whether a given such Stein manifold is symplectomorphic to the first one or not. We prove a similar undecidability result for contact structures on the 2n - 1 dimensional sphere. We can generalize these results by replacing com- plex affine space with any smooth affine variety of dimension n and the 2n - 1 dimensional sphere with any smooth affine variety intersected with a sufficiently large sphere. We prove these theorems by using an invariant called the growth rate of symplectic homology to reduce these problems to an undecidability result for groups. 
Mark McLean
math.SG