Differentiable Actions on Homotopy Seven Spheres

摘要

This report presents studies of differentiable actions of the circle group SO(2) on homotopy 7-spheres. The class of free such actions is shown to form an infinite abelian group which is closely related to the group of differentiable knotted 3-spheres in the 6-sphere. Studies are also presented on actions, each having a 3-sphere as the fixed point set and acting freely otherwise. (Author)