@article{KorecZnam1987,
  author    = {{Ivan  Korec and Stefan Znam}},
  title     = {{A Note on the $3x  +  1$ Problem}},
  journal   = {{Amer. Math. Monthly}},
  year      = {{1987}},
  volume    = {{94}},
  pages     = {{771--772}},
  note      = {{MR 90g:11023}},
}