Published online by Cambridge University Press: 20 November 2018
In the finite von Neumann algebra setting, we introduce the concept of a perturbation determinant associated with a pair of self-adjoint elements  
 ${{H}_{0}}$  and  
 $H$  in the algebra and relate it to the concept of the de la Harpe–Skandalis homotopy invariant determinant associated with piecewise  
 ${{C}^{1}}$ -paths of operators joining  
 ${{H}_{0}}$  and  
 $H$ . We obtain an analog of Krein's formula that relates the perturbation determinant and the spectral shift function and, based on this relation, we derive subsequently (i) the Birman–Solomyak formula for a general non-linear perturbation, (ii) a universality of a spectral averaging, and (iii) a generalization of the Dixmier–Fuglede–Kadison differentiation formula.