@article{MOMA_1980__90_177988,
     author = {J\"urgen Voigt},
     title = {A {Perturbation} {Theorem} for the {Essential} {Spectral} {Radius} of {Strongly} {Continuous} {Semigroups.}},
     journal = {Monatshefte f\"ur Mathematik},
     pages = {153--162},
     publisher = {mathdoc},
     volume = {90},
     year = {1980},
     zbl = {0433.47022},
     url = {http://geodesic.mathdoc.fr/item/MOMA_1980__90_177988/}
}