@article{Li2005,
  author    = {{Li, Xiao Chun}},
  title     = {{Necessary condition for periodic numbers in the $3N+1$ conjecture}},
  journal   = {{J. Huazhong Univ. Sci. Technol. Nat. Sci.}},
  year      = {{2005}},
  volume    = {{33}},
  number    = {{11}},
  pages     = {{102--103}},
  note      = {{MR 2209315}},
}