@unpublished{GalstianYagdjian2008,
  author  = {Anahit Galstian and Karen Yagdjian},
  title   = {Exponential function of pseudo-differential operators},
  institution = {Institut f{\"u}r Mathematik},
  series    = {Preprint},
  number    = {(1997) 13},
  year    = {2008},
}