@book{norlund:1924:vdr,
    author = {N\"orlund, Niels Erik},
     title = {{V}orlesungen \"uber {D}ifferenzenrechnung},
 publisher = {Springer-Verlag},
   address = {Berlin},
      year = {1924},
  language = {German},
      note = {Reprinted 1954 by Chelsea Publishing Company, New York},
     zblno = {50.0315.02}}