1. Trotsenko D. A. and Väisälä J., “Upper sets and quasisymmetric maps,” Ann. Acad. Sci. Fenn. Ser. A I Math., 24, No. 2, 465–488 (1999).
2. Trotsenko D. A., “Gromov hyperbolic discrete spaces and their application to extension of classes of mappings,” Dokl. Math., 438, No. 3, 344–347 (2011).
3. Trotsenko D. A., “Extendability of classes of maps and new properties of upper sets,” Complex Anal. Operator Theory, 5, No. 3, 967–984 (2011).
4. Pommerenke C., “Uniformly perfect sets and the Poincar´e metric,” Arch. Math. (Basel), 32, 192–199 (1979).
5. Järvi P. and Vuorinen M., “Uniformly perfect sets and quasiregular mappings,” J. London Math. Soc., 54, No. 174. Part 3, 515–529 (1996).