1. Guo, D, Lakshmikantham, V: Coupled fixed points of nonlinear operators with applications. Nonlinear Anal. 11, 623-632 (1987)
2. SAFA;H Amann,1977
3. Bota, M-F, Petruşel, A, Petruşel, G, Samet, B: Coupled fixed point theorems for singlevalued operators in b-metric spaces. Fixed Point Theory Appl. 2015, 231 (2015)
4. Fréchet, M: Les espaces abstraits. Gauthier-Villars, Paris (1928)
5. Rus, IA: Picard operators and applications. Sci. Math. Jpn. 58, 191-219 (2003)