Spheres with more than 7 vector fields: all the fault of Spin(9)

Abstract

We give an interpretation of the maximal number of linearly independent vector fields on spheres in terms of the Spin(9) representation on R^16. This casts an insight on the role of Spin(9) as a subgroup of SO(16) on the existence of vector fields on spheres, parallel to the one played by complex, quaternionic and octonionic structures on R^2, R^4 and R^8, respectively.Comment: 14 pages. Revised version. The proof of the main theorem, now without induction, is in the completely new Section 6. Notations have been simplified in all the pape