[1] H. Amann, On the number of solutions of nonlinear equations in
ordered Banach spaces, J. Funct. Anal. 11 (1972), 346-384.
[2] Yihong Du, Fixed points of increasing operators in ordered Banach
spaces and applications, Appl. Anal. 38 (1990), 1-20.
[3] Dajun Ge and V. Lakshmikantham, Couple fixed points of nonlinear
operators with applications, Nonlinear Anal. TMA 11 (1987),
623-632.
[4] Dajun Ge, Fixed points of mixed monotone operators with
applications, Appl. Anal. 31 (1988), 215-224.