In mathematics, a pseudometric space is a generalization of a metric space in which the distance between two distinct points can be zero. Pseudometric spaces were introduced by Đuro Kurepa in 1934. 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
Any metric space is a pseudometric space. 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
A seminorm induces the pseudometric . This is a convex function of an affine function of (in particular, a translation), and therefore convex in . (Likewise for .)
Conversely, a homogeneous, translation-invariant pseudometric induces a seminorm.
Pseudometrics also arise in the theory of hyperbolic complex manifolds: see Kobayashi metric.
Every measure space can be viewed as a complete pseudometric space by defining
If is a function and d2 is a pseudometric on X2, then gives a pseudometric on X1. If d2 is a metric and f is injective, then d1 is a metric.
The pseudometric topology is the topology generated by the open balls
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 (that is, distinct points are topologically distinguishable).
The definitions of Cauchy sequences and metric completion for metric spaces carry over to pseudometric spaces unchanged.
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 be the quotient space of by this equivalence relation and define
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 and is saturated. The topological identification is the Kolmogorov quotient.
An example of this construction is the completion of a metric space by its Cauchy sequences.
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 .