1. Khan MS, Swaleh M, Sessa S: Fixed point theorems by altering distances between the points. Bull. Aust. Math. Soc. 1984, 30: 1–9. 10.1017/S0004972700001659
2. Reich S: Some fixed point problems. Atti Accad. Naz. Lincei, Rend. Cl. Sci. Fis. Mat. Nat. 1974, 57: 194–198.
3. Babu GVR, Lalitha B, Sandhya ML: Common fixed point theorems involving two generalized altering distance functions in four variables. Proc. Jangjeon Math. Soc. 2007, 10: 83–93.
4. Caristi J: Fixed point theorems for mapping satisfying inwardness conditions. Trans. Am. Math. Soc. 1976, 215: 241–251.
5. Dutta PN, Choudhury BS: A generalization of contraction principle in metric spaces. Fixed Point Theory Appl. 2008., 2008: Article ID 406368