Unbounded Fredholm Operators and Spectral Flow

We study the gap (= “projection norm” = “graph distance”) topology of the space of (not necessarily bounded) self–adjoint Fredholm operators in a separable Hilbert space by the Cayley transform and direct methods. In particular, we show that the space is connected contrary to the bounded case. Moreover, we present a rigorous definition of spectral flow of a path of such operators (actually alternative but mutually equivalent definitions) and prove the homotopy invariance. As an example, we discuss operator curves on manifolds with boundary.