In mathematics, a pseudometric or semi-metric space is a generalized metric space in which the distance between two distinct points can be zero. In the same way as every normed space is a metric space, every seminormed space is a pseudometric space. Because of this analogy the term semimetric space (which has a different meaning in topology) is sometimes used as a synonym, especially in functional analysis.
When a topology is generated using a family of pseudometrics, the space is called a gauge space.
A pseudometric space is a set together with a non-negative real-valued function (called a pseudometric) such that, for every ,
Unlike a metric space, points in a pseudometric space need not be distinguishable; that is, one may have for distinct values .
- Pseudometrics arise naturally in functional analysis. Consider the space of real-valued functions together with a special point . This point then induces a pseudometric on the space of functions, given by
- For vector spaces , a seminorm induces a pseudometric on , as
- Conversely, a homogeneous, translation invariant pseudometric induces a seminorm.
- Every measure space can be viewed as a complete pseudometric space by defining
- for all .
which form a basis for the topology. A topological space is said to be a pseudometrizable topological space if the space can be given a pseudometric such that the pseudometric topology coincides with the given topology on the space.
The difference between pseudometrics and metrics is entirely topological. That is, a pseudometric is a metric if and only if the topology it generates is T0 (i.e. distinct points are topologically distinguishable).
The vanishing of the pseudometric induces an equivalence relation, called the metric identification, that converts the pseudometric space into a full-fledged metric space. This is done by defining if . Let and let
Then is a metric on and is a well-defined metric space.
The metric identification preserves the induced topologies. That is, a subset is open (or closed) in if and only if is open (or closed) in . The topological identification is the Kolmogorov quotient.
- Dmitri Burago, Yu D Burago, Sergei Ivanov, A Course in Metric Geometry, American Mathematical Society, 2001, ISBN 0-8218-2129-6.
- Pseudometric topology at PlanetMath.org.
- Howes, Norman R. (1995). Modern Analysis and Topology. New York, NY: Springer. p. 27. ISBN 0-387-97986-7. Retrieved 10 September 2012. "Let be a pseudo-metric space and define an equivalence relation in by if . Let be the quotient space and the canonical projection that maps each point of onto the equivalence class that contains it. Define the metric in by for each pair . It is easily shown that is indeed a metric and defines the quotient topology on ."
- Arkhangel'skii, A.V.; Pontryagin, L.S. (1990). General Topology I: Basic Concepts and Constructions Dimension Theory. Encyclopaedia of Mathematical Sciences. Springer. ISBN 3-540-18178-4.
- Steen, Lynn Arthur; Seebach, Arthur (1995) . Counterexamples in Topology (new edition ed.). Dover Publications. ISBN 0-486-68735-X.
- This article incorporates material from Pseudometric space on PlanetMath, which is licensed under the Creative Commons Attribution/Share-Alike License.
- Example of pseudometric space at PlanetMath.org.