@article{Lehtonen2008,
  author    = {{Eero Lehtonen}},
  title     = {{Two undecidable versions of Collatz's problems}},
  journal   = {{Theor. Comp. Sci.}},
  year      = {{2008}},
  volume    = {{407}},
  pages     = {{596--600}},
  note      = {{MR 2009k:68092}},
}