@article{Lagarias1990,
  author    = {{Jeffrey C. Lagarias}},
  title     = {{The set of rational cycles for the $3x  +  1$ problem}},
  journal   = {{Acta Arithmetica}},
  year      = {{1990}},
  volume    = {{56}},
  pages     = {{33--53}},
  note      = {{MR 91i:11024}},
}