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@article{SF_1986__34_134849, author = {J.A. Goldstein and S. \^Oharu}, title = {A remark on nonlinear cosine functions.}, journal = {Semigroup forum}, pages = {359--366}, publisher = {mathdoc}, volume = {34}, year = {1986}, zbl = {0609.47075}, url = {http://geodesic.mathdoc.fr/item/SF_1986__34_134849/} }
J.A. Goldstein; S. Ôharu. A remark on nonlinear cosine functions.. Semigroup forum, Tome 34 (1986), pp. 359-366. http://geodesic.mathdoc.fr/item/SF_1986__34_134849/