讲座内容：

Spectral flow, for a norm continuous path of self adjoint Fredholm operators,  roughly measures the ''net mass" of the part of the spectrum that passes zero, as we move along the path.  We discuss the definition of spectral flow, including an interesting projection lifting hypothesis that appears in all treatments.  Our first main result is that spectral flow induces a group isomorphism pi_1(F_[SA, infinity]) rightarrow K_0(B).  Our second main result is an axiomatization of Spectral flow.