@unpublished{SchulzeNazaikinskiiSternin2008,
  author  = {Bert-Wolfgang Schulze and Vladimir E. Nazaikinskii and Boris Sternin},
  title   = {On the homotopy classification of elliptic operators on manifolds with singularities},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(1999) 21},
  year    = {2008},
}