@unpublished{ShlapunovTarkhanov2009,
  author  = {Alexander Shlapunov and Nikolai Nikolaevich Tarkhanov},
  title   = {Formal Poincar{\´e} lemma},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(2007) 02},
  year    = {2009},
}