@unpublished{BaerBallmann2012,
  author  = {Christian B{\"a}r and Werner Ballmann},
  title   = {Boundary value problems for elliptic differential operators of first order},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprints des Instituts f{\"u}r Mathematik der Universit{\"a}t Potsdam},
  number    = {1(2012)18},
  year    = {2012},
}