References
- Guo , Dajun and Lakshmikantham , V. 1987 . Coupled fixed points of nonlinear operators with applications . Nonlinear Anal TMA , 11 : 623 – 632 .
- Guo , Dajun and Lakshmikantham , V. 1966 . Nonlinear problems in abstract cones , Boston & New York : Academic Press,Inc .