1. A note on contractive mappings;Rakotch,1962
2. Sugli elementi uniti delle transformazioin funzionali;Caccioppoli,1932
3. Metric and Normed Spaces;Kolmogorov,1957
4. Remarks on a generalization of Banach's principle of contraction;Chu;J. Math. Anal. Appl.,1965
5. On Lipschitz structures;Geraghty;J. Math. Anal. Appl.,1965