@article{Sander1990,
  author    = {{Jürgen W. Sander}},
  title     = {{On the $(3N  +  1)$-conjecture}},
  journal   = {{Acta Arithmetica}},
  year      = {{1990}},
  volume    = {{55}},
  pages     = {{241--248}},
  note      = {{MR 91m:11052}},
}