@unpublished{CalinDerChen2008,
  author  = {Ovidium Calin and Chang Der-Chen},
  title   = {The geometry on a step 3 Grushin model},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(2004) 08},
  year    = {2008},
}