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 n≠4, 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?