@article{Barmpalias2006IbtDegreesC,
  title   = {The ibT degrees of computably enumerable sets are not dense},
  journal = {Ann. Pure Appl. Logic},
  year    = {2006},
  volume  = {141},
  number  = {1-2},
  pages   = {51-60},
  author  = {G. Barmpalias and A. E. M. Lewis},
}