@article{SF_1995__51_1_135411,
     author = {S. Piskarev and S.YX. Shaw},
     title = {Pertubation and comparison of cosine operator functions.},
     journal = {Semigroup forum},
     pages = {225--246},
     publisher = {mathdoc},
     volume = {51},
     number = {1},
     year = {1995},
     zbl = {0828.47037},
     url = {http://geodesic.mathdoc.fr/item/SF_1995__51_1_135411/}
}