Nagata–Smirnov metrization theorem
The Nagata–Smirnov metrization theorem in topology characterizes when a topological space is metrizable. The theorem states that a topological space 
is metrizable if and only if it is regular, Hausdorff and has a countably locally finite (i.e., σ-locally finite) basis.
A topological space X is called a regular space if every non-empty closed subset C of X and a point p not contained in C admit non-overlapping open neighborhoods. A collection in a space X is countably locally finite (or σ-locally finite) if it is the union of a countable family of locally finite collections of subsets of X.
Unlike Urysohn's metrization theorem, which provides only a sufficient condition for metrizability, this theorem provides both a necessary and sufficient condition for a topological space to be metrizable. The theorem is named after Junichi Nagata and Yuriĭ Mikhaĭlovich Smirnov.
See also
References
- Munkres, James R. (1975), "Sections 6-2 and 6-3", Topology, Prentice Hall, pp. 247–253, ISBN 0-13-925495-1.
- Patty, C. Wayne (2009), "7.3 The Nagata–Smirnov Metrization Theorem", Foundations of Topology (2nd ed.), Jones & Bartlett, pp. 257–262, ISBN 978-0-7637-4234-8.