References
- Lakshmikantham , V. and Leela , S. 1981 . Nonlinear Differential Equation in Abstract Spaces , Pergamon Press .
- Yosida , K. 1980 . Functional Analysis , Springer-Verlag .
- Lakshmikantham , V. and Leela , S. 1969 . Differential and integralinequalities , Vol. 1 , Academic Press .