@unpublished{KurthsVossWittetal.2007,
  author  = {J{\"u}rgen Kurths and A. Voss and Annette Witt and P. Saparin and H. J. Kleiner and Niels Wessel},
  title   = {Quantitative analysis of heart rate variability},
  institution = {Institut f{\"u}r Physik und Astronomie},
  series    = {NLD Preprints},
  number    = {5},
  year    = {2007},
}