@unpublished{PfaeffleStephan2012,
  author  = {Frank Pf{\"a}ffle and Christoph A. Stephan},
  title   = {The Holst action by the spectral action principle},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprints des Instituts f{\"u}r Mathematik der Universit{\"a}t Potsdam},
  number    = {1(2012)19},
  year    = {2012},
}