@article{Wirsching2003,
  author    = {{Wirsching, Günther J.}},
  title     = {{On the problem of positive predecessor density in $3N+1$ dynamics}},
  journal   = {{Disc. Cont. Dynam. Syst.}},
  year      = {{2003}},
  volume    = {{9}},
  pages     = {{771--787}},
  note      = {{MR 2004f:39028}},
}