@article{MAN_1977__229_163012,
     author = {J\"urgen Voigt},
     title = {On the {Perturbation} {Theory} for {Strongly} {Continuous} {Semigroups.}},
     journal = {Mathematische Annalen},
     pages = {163--172},
     publisher = {mathdoc},
     volume = {229},
     year = {1977},
     zbl = {0338.47018},
     url = {http://geodesic.mathdoc.fr/item/MAN_1977__229_163012/}
}