1. K. Yosida,Functional Analysis, Springer-Verlag, Berlin-New York (1980).
2. S. G. Krein,Linear Differential Equations in Banach Space, Amer. Math. Soc., Providence, Rhode Island (1972).
3. S. G. Krein and M. I. Hazan, “Differential equations in a Banach space,” In:Progress in Science and Technology. Series on Math. Anal. Vol. 21,Itogi Nauki i Tekhniki, VINITI, Moscow (1983), pp. 130–232.
4. I. V. Melnikova,Operator-functions εC(t), εS(t) and well-posedness of the Cauchy problem for the second-order equation, Deposited at the All-Union Institute for Scientific and Technical Information (1982).
5. I. V. Melnikova and A. I. Filinkov, “Classification and well-posedness of the Cauchy problem for second-order equations in a Banach space,”Dokl. Akad. Nauk SSSR,276, 1066–1071 (1984); English transl. inSov. Math. Dokl.,29 (1984).