@article{M2AN_1981__15_3_231_0,
     author = {Helfrich, Hans-Peter},
     title = {Simultaneous approximation in negative norms of arbitrary order},
     journal = {ESAIM: Mathematical Modelling and Numerical Analysis },
     pages = {231--235},
     year = {1981},
     publisher = {Centrale des revues, Dunod-Gauthier-Villars},
     address = {Montreuil},
     volume = {15},
     number = {3},
     mrnumber = {631677},
     zbl = {0495.41010},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/M2AN_1981__15_3_231_0/}
}