@article{Wirsching1996,
  author    = {{Günther J. Wirsching}},
  title     = {{On the Combinatorial Structure of $3N+1$ Predecessor Sets}},
  journal   = {{Discrete Math.}},
  year      = {{1996}},
  volume    = {{148}},
  pages     = {{265--286}},
  note      = {{MR 97b:11029}},
}