@article{ApplegateLagarias1995a,
  author    = {{David Applegate and Jeffrey C. Lagarias}},
  title     = {{Density Bounds for the $3x+1$ Problem I. Tree-Search Method}},
  journal   = {{Math. Comp.}},
  year      = {{1995}},
  volume    = {{64}},
  pages     = {{411--426}},
  note      = {{MR 95c:11024}},
}