@unpublished{KrupchykTarkhanovTuomela2009,
  author  = {K. Krupchyk and Nikolai Nikolaevich Tarkhanov and J. Tuomela},
  title   = {Generalised elliptic boundary problems},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(2005) 23},
  year    = {2009},
}