@article{Krasikov1989,
  author    = {{Ilia Krasikov}},
  title     = {{How many numbers satisfy the $3x  +  1$ Conjecture?}},
  journal   = {{Internatl. J. Math. & Math. Sci.}},
  year      = {{1989}},
  volume    = {{12}},
  pages     = {{791--796}},
  note      = {{MR 91c:11013}},
}