smooth structures

Relation to homotopy: Define a monoid Gn with

  • Objects: smooth structures on the n sphere (identified as oriented smooth n-manifolds which are homeomorphic to Sn)
  • Binary operation: Connect sum

For n4, this is a group. Turns out to be isomorphic to Θn, the group of h-cobordism classes of “homotopy Sns”

Recently (almost) resolved question: what is Θn for all n?

Application: what spheres admit unique smooth structures?