1. Fattorini, H.O., Ordinary Differential Equations in Linear Topological Space. II, J. Differential Equations, 1969, vol. 6, pp. 50–70.
2. Sova, M., Cosine Operator Functions, Rozpr. mat., 1966, no. 49, pp. 1–47.
3. Goldstein, J.A., Semigroups of Linear Operators and Applications, New York: Oxford Univ., 1985. Translated under the title Polugruppy lineinykh operatorov i ikh prilozheniya, Kiev: Vyshcha Shkola, 1989.
4. Vasil’ev, V.V., Krein, S.G., and Piskarev, S.I., Operator Semigroups, Cosine Operator Functions, and Linear Differential Equations, Itogi Nauki i Tekhn. Mat. Analiz, 1990, vol. 28, pp. 87–202.
5. Vasil’ev, V.V. and Piskarev, S.I., Differential Equations in Banach Space. II. Theory of Cosine Operator Functions, http://www.srcc.msu.su/nivc/english/about/home pages/piskarev/obz2ru.pdf