@article{SF_1995__51_1_135420,
     author = {W.M. Ruess},
     title = {Purely imaginary eigenvalues of operator semigroups.},
     journal = {Semigroup forum},
     pages = {335--342},
     publisher = {mathdoc},
     volume = {51},
     number = {1},
     year = {1995},
     zbl = {0831.47031},
     url = {http://geodesic.mathdoc.fr/item/SF_1995__51_1_135420/}
}