1. Bourbaki N. Topologie generale herman. Paris, France.
2. The contraction mapping principle in almost metric spaces;Bakhtin;Funct Anal,1989
3. Nonlinear set-valued contraction mappings in b-metric spaces;Czerwik;Atti Semin Mat Fis Univ Modena,1998
4. On semi-metric spaces;Wilson;Am J Math,1931
5. Axioms that define semi-metric, Moore and metric spaces;Boyd;Proc Am Math Soc,1962