@article{Muller1991,
  author    = {{Helmut A. Müller}},
  title     = {{Das `$3n+1$' Problem}},
  journal   = {{Mitteilungen der Math. Ges. Hamburg}},
  year      = {{1991}},
  volume    = {{12}},
  pages     = {{231--251}},
  note      = {{MR 93c:11053}},
}