1. Positive Solutions of Differential, Difference and Integral Equations;Agarwal,1999
2. Volterra Integral and Differential Equations;Burton,1983
3. J. Goldstein, Semigroup of Linear Operators and Applications, Oxford, New York, 1985.
4. Monotone Iterative Techniques for Discontinuous Nonlinear Differential Equations;Heikkila,1994
5. Existence Theory for Nonlinear Integral and Integrodifferential Equations;O’Regan,1998