1. R. P. Agarwal, D. O'Regan; . Appl. Math. Lett., 13(4) (2000), 7-11.
2. R. P. Agarwal, D. O'Regan; , Nonlinear Anal., 44 (2001), 527-535.
3. R. P. Agarwal, M. Bohner, P. Wong; . Apply. Math. Comput., 99 (1999), 153-166.
4. D. R. Anderson; , Advances in Dynamical Systems and Applications, 1:1 (2006), 17-27.
5. B. Aulbach, S. Hilger; , Nonlinear Dyn. Quantum Dyn. Sys., (Gaussig, 1990) volume 59 of Math. Res., 9-20. Akademie Verlag, Berlin, 1990.