Tijun, Xiao, Second Order Linear Differential Equa- tions with Almost Periodic Solutions, Acta Mathematica Sinica, New Series Chinese Journal of Mathematics, 7(4), 1991, pp. 354-359
In this paper a characterization of almost periodic strongly continuous Sine operator function is given, and in a Hilbert space. it is shown that the almost periodicity of a Sine operator function implies that of the corresponding Cosine operator function.