@unpublished{KellerRoellyValleriani2012,
  author  = {Peter Keller and Sylvie Roelly and Angelo Valleriani},
  title   = {On time duality for quasi-birth-and-death processes},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprints des Instituts f{\"u}r Mathematik der Universit{\"a}t Potsdam},
  number    = {1 (2012) 4},
  year    = {2012},
}