@article{ASNSP_1997_4_24_4_703_0,
     author = {Nagel, Rainer},
     title = {Characteristic equations for the spectrum of generators},
     journal = {Annali della Scuola Normale Superiore di Pisa - Classe di Scienze},
     pages = {703--717},
     publisher = {Scuola normale superiore},
     volume = {Ser. 4, 24},
     number = {4},
     year = {1997},
     mrnumber = {1627330},
     zbl = {0912.47023},
     language = {en},
     url = {http://www.numdam.org/item/ASNSP_1997_4_24_4_703_0/}
}