@article{MZ_1981__176_183658,
     author = {Rainer Colgen},
     title = {Invariance of {Essential} {Spectra.}},
     journal = {Mathematische Zeitschrift},
     pages = {489--494},
     publisher = {mathdoc},
     volume = {176},
     year = {1981},
     zbl = {0439.47007},
     url = {http://geodesic.mathdoc.fr/item/MZ_1981__176_183658/}
}