Bernhelm Booß-Bavnbek, Krzysztof P. Wojciechowski: The Homotopy Groups of the Space of Self-Adjoint Fredholm Operators, §16 in: Elliptic Boundary Problems for Dirac Operators Mathematics: Theory & Applications, Birkhäuser (1993) 127-137 [doi:10.1007/978-1-4612-0337-7_16, pdf]