@article{SF_1978__16_134276,
     author = {J. Goldstein and c. Radin and R. Showalter},
     title = {Convergence rates of ergodic limits for semigroups and cosine functions.},
     journal = {Semigroup forum},
     pages = {89--96},
     year = {1978},
     volume = {16},
     zbl = {0393.47004},
     url = {http://geodesic.mathdoc.fr/item/SF_1978__16_134276/}
}