@article{Wirsching1993,
  author    = {{Günther  J. Wirsching}},
  title     = {{An Improved Estimate Concerning $3N  +  1$ Predecessor Sets}},
  journal   = {{Acta Arithmetica}},
  year      = {{1993}},
  volume    = {{63}},
  pages     = {{205--210}},
  note      = {{MR 94e:11018}},
}