@article{Luca2005,
  author    = {{Luca, Florian}},
  title     = {{On the nontrivial cycles in Collatz's problem}},
  journal   = {{SUT Journal of Mathematics}},
  year      = {{2005}},
  volume    = {{41}},
  number    = {{1}},
  pages     = {{31--41}},
  note      = {{MR 2006e:11034}},
}