Perturbative method for generalized spectral decompositions
  Imposing analytic properties to states and observables we construct a perturbative method to obtain a generalized biorthogonal system of eigenvalues and eigenvectors for quantum unstable systems. A decay process can be described using this generalized spectral decomposition, and the final generalized state is obtained. 
Roberto Laura, Mario Castagnino and Rodolfo M. Id Betan