1. Urysohn P. S., “Beispiel eines nirgends separablen metrischen Raumes,” Fund. Math., vol. 9, 119–121 (1927).
2. Urysohn P. S., “An example of a metric space nowhere satisfying the second countability axiom,” in: Urysohn P. S., Works on Topology and Other Areas of Mathematics. Vol. 2 [Russian], GITTL, Moscow and Leningrad, 1951, 778–780.
3. Kuratowski K., Topology. Vol. 1, Academic Press and Polish Scientific Publishers, New York, London, and Warszawa (1966).
4. Engelking R., General Topology, Heldermann Verlag, Berlin (1989).
5. Dyubina A. G. and Polterovich I. V., “The structure of hyperbolic spaces at infinity,” Russian Math. Surveys, vol. 53, No. 5, 1093–1094 (1998).