@article{BohmSontacchi1978,
  author    = {{Corrado Böhm and Giovanna Sontacchi}},
  title     = {{On the existence of cycles of given length in integer sequences like $x_{n+1}= x_n/2$ if $x_n$ even, and $x_{n+1}= 3x_n + 1$ otherwise}},
  journal   = {{Atti Accad. Naz. Lincei Rend. Cl. Sci. Fis. Mat. Natur.}},
  year      = {{1978}},
  volume    = {{64}},
  pages     = {{260--264}},
  url       = {{http://www.bdim.eu/item?id=RLINA_1978_8_64_3_260_0&fmt=pdf}},
  note      = {{MR 83h:10030}},
}