@article{ASNSP_1993_4_20_3_357_0,
     author = {Cicognani, Massimo and Zanghirati, Luisa},
     title = {On a class of unsolvable operators},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {357--369},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 20},
     number = {3},
     year = {1993},
     mrnumber = {1256073},
     zbl = {0816.47051},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1993_4_20_3_357_0/}
}