@unpublished{SavinSchulzeSternin2008,
  author  = {Anton Savin and Bert-Wolfgang Schulze and Boris Sternin},
  title   = {On the invariant index formulas for spectral boundary value problems},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(1998) 18},
  year    = {2008},
}