Metric (mathematics)
In mathematics, a metric or distance function is a function that defines a distance between each pair of elements of a set. A set with a metric is called a metric space. A metric induces a topology on a set, but not all topologies can be generated by a metric. A topological space whose topology can be described by a metric is called metrizable.
An important source of metrics in differential geometry are metric tensors, bilinear forms that may be defined from the tangent vectors of a differentiable manifold onto a scalar. A metric tensor allows distances along curves to be determined through integration, and thus determines a metric. However, not every metric comes from a metric tensor in this way.
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Definition
A metric on a set X is a function (called the distance function or simply distance)
 ,
where is the set of nonnegative real numbers and for all , the following conditions are satisfied:
1. nonnegativity or separation axiom 2. identity of indiscernibles 3. symmetry 4. subadditivity or triangle inequality
Conditions 1 and 2 together define a positivedefinite function. The first condition is implied by the others.
A metric is called an ultrametric if it satisfies the following stronger version of the triangle inequality where points can never fall 'between' other points:
for all
A metric d on X is called intrinsic if any two points x and y in X can be joined by a curve with length arbitrarily close to d(x, y).
For sets on which an addition + : X × X → X is defined, d is called a translation invariant metric if
for all x, y, and a in X.
Notes
These conditions express intuitive notions about the concept of distance. For example, that the distance between distinct points is positive and the distance from x to y is the same as the distance from y to x. The triangle inequality means that the distance from x to z via y is at least as great as from x to z directly. Euclid in his work stated that the shortest distance between two points is a line; that was the triangle inequality for his geometry.
If a modification of the triangle inequality
 4*. d(x, z) ≤ d(z, y) + d(y, x)
is used in the definition then property 1 follows straight from property 4*. Properties 2 and 4* give property 3 which in turn gives property 4.
Examples
 The discrete metric: if x = y then d(x,y) = 0. Otherwise, d(x,y) = 1.
 The Euclidean metric is translation and rotation invariant.
 The taxicab metric is translation invariant.
 More generally, any metric induced by a norm is translation invariant.
 If is a sequence of seminorms defining a (locally convex) topological vector space E, then
 is a metric defining the same topology. (One can replace by any summable sequence of strictly positive numbers.)
 Graph metric, a metric defined in terms of distances in a certain graph.
 The Hamming distance in coding theory.
 Riemannian metric, a type of metric function that is appropriate to impose on any differentiable manifold. For any such manifold, one chooses at each point p a symmetric, positive definite, bilinear form L: T_{p} × T_{p} → ℝ on the tangent space T_{p} at p, doing so in a smooth manner. This form determines the length of any tangent vector v on the manifold, via the definition v = √L(v, v). Then for any differentiable path on the manifold, its length is defined as the integral of the length of the tangent vector to the path at any point, where the integration is done with respect to the path parameter. Finally, to get a metric defined on any pair {x, y} of points of the manifold, one takes the infimum, over all paths from x to y, of the set of path lengths. A smooth manifold equipped with a Riemannian metric is called a Riemannian manifold.
 The Fubini–Study metric on complex projective space. This is an example of a Riemannian metric.
 String metrics, such as Levenshtein distance and other string edit distances, define a metric over strings.
 Graph edit distance defines a distance function between graphs.
 The Wasserstein metric is a distance function defined between two probability distributions.
Equivalence of metrics
For a given set X, two metrics d_{1} and d_{2} are called topologically equivalent (uniformly equivalent) if the identity mapping
 id: (X,d_{1}) → (X,d_{2})
is a homeomorphism (uniform isomorphism).
For example, if is a metric, then and are metrics equivalent to
See also notions of metric space equivalence.
Metrics on vector spaces
Norms on vector spaces are equivalent to certain metrics, namely homogeneous, translationinvariant ones. In other words, every norm determines a metric, and some metrics determine a norm.
Given a normed vector space we can define a metric on X by
 .
The metric d is said to be induced by the norm .
Conversely if a metric d on a vector space X satisfies the properties
 (translation invariance)
 (homogeneity)
then we can define a norm on X by
Similarly, a seminorm induces a pseudometric (see below), and a homogeneous, translation invariant pseudometric induces a seminorm.
Metrics on multisets
We can generalize the notion of a metric from a distance between two elements to a distance between two nonempty finite multisets of elements. A multiset is a generalization of the notion of a set such that an element can occur more than once. Define if is the multiset consisting of the elements of the multisets and , that is, if occurs once in and once in then it occurs twice in . A distance function on the set of nonempty finite multisets is a metric^{[1]} if
 if all elements of are equal and otherwise (positive definiteness), that is, (nonnegativity plus identity of indiscernibles)
 is invariant under all permutations of (symmetry)
 (triangle inequality)
Note that the familiar metric between two elements results if the multiset has two elements in 1 and 2 and the multisets have one element each in 3. For instance if consists of two occurrences of , then according to 1.
A simple example is the set of all nonempty finite multisets of integers with . More complex examples are information distance in multisets;^{[1]} and normalized compression distance (NCD) in multisets.^{[2]}
Generalized metrics
There are numerous ways of relaxing the axioms of metrics, giving rise to various notions of generalized metric spaces. These generalizations can also be combined. The terminology used to describe them is not completely standardized. Most notably, in functional analysis pseudometrics often come from seminorms on vector spaces, and so it is natural to call them "semimetrics". This conflicts with the use of the term in topology.
Extended metrics
Some authors allow the distance function d to attain the value ∞, i.e. distances are nonnegative numbers on the extended real number line. Such a function is called an extended metric or "∞metric". Every extended metric can be transformed to a finite metric such that the metric spaces are equivalent as far as notions of topology (such as continuity or convergence) are concerned. This can be done using a subadditive monotonically increasing bounded function which is zero at zero, e.g. d′(x, y) = d(x, y) / (1 + d(x, y)) or d′′(x, y) = min(1, d(x, y)).
The requirement that the metric take values in [0,∞) can even be relaxed to consider metrics with values in other directed sets. The reformulation of the axioms in this case leads to the construction of uniform spaces: topological spaces with an abstract structure enabling one to compare the local topologies of different points.
Pseudometrics
A pseudometric on X is a function d : X × X → R which satisfies the axioms for a metric, except that instead of the second (identity of indiscernibles) only d(x,x)=0 for all x is required. In other words, the axioms for a pseudometric are:
 d(x, y) ≥ 0
 d(x, x) = 0 (but possibly d(x, y) = 0 for some distinct values x ≠ y.)
 d(x, y) = d(y, x)
 d(x, z) ≤ d(x, y) + d(y, z).
In some contexts, pseudometrics are referred to as semimetrics because of their relation to seminorms.
Quasimetrics
Occasionally, a quasimetric is defined as a function that satisfies all axioms for a metric with the possible exception of symmetry:^{[3]}^{[4]}. The name of this generalisation is not entirely standardized.^{[5]}
 d(x, y) ≥ 0 (positivity)
 d(x, y) = 0 if and only if x = y (positive definiteness)

d(x, y) = d(y, x)(symmetry, dropped)  d(x, z) ≤ d(x, y) + d(y, z) (triangle inequality)
Quasimetrics are common in real life. For example, given a set X of mountain villages, the typical walking times between elements of X form a quasimetric because travel up hill takes longer than travel down hill. Another example is a taxicab geometry topology having oneway streets, where a path from point A to point B comprises a different set of streets than a path from B to A.
A quasimetric on the reals can be defined by setting
 d(x, y) = x − y if x ≥ y, and
 d(x, y) = 1 otherwise. The 1 may be replaced by infinity or by .
The topological space underlying this quasimetric space is the Sorgenfrey line. This space describes the process of filing down a metal stick: it is easy to reduce its size, but it is difficult or impossible to grow it.
If d is a quasimetric on X, a metric d' on X can be formed by taking
 d'(x, y) = ^{1}⁄_{2}(d(x, y) + d(y, x)).
Metametrics
In a metametric, all the axioms of a metric are satisfied except that the distance between identical points is not necessarily zero. In other words, the axioms for a metametric are:
 d(x, y) ≥ 0
 d(x, y) = 0 implies x = y (but not vice versa.)
 d(x, y) = d(y, x)
 d(x, z) ≤ d(x, y) + d(y, z).
Metametrics appear in the study of Gromov hyperbolic metric spaces and their boundaries. The visual metametric on such a space satisfies d(x, x) = 0 for points x on the boundary, but otherwise d(x, x) is approximately the distance from x to the boundary. Metametrics were first defined by Jussi Väisälä.^{[6]}
Semimetrics
A semimetric on X is a function d : X × X → R that satisfies the first three axioms, but not necessarily the triangle inequality:
 d(x, y) ≥ 0
 d(x, y) = 0 if and only if x = y
 d(x, y) = d(y, x)
Some authors work with a weaker form of the triangle inequality, such as:
 d(x, z) ≤ ρ (d(x, y) + d(y, z)) (ρrelaxed triangle inequality)
 d(x, z) ≤ ρ max(d(x, y), d(y, z)) (ρinframetric inequality).
The ρinframetric inequality implies the ρrelaxed triangle inequality (assuming the first axiom), and the ρrelaxed triangle inequality implies the 2ρinframetric inequality. Semimetrics satisfying these equivalent conditions have sometimes been referred to as "quasimetrics",^{[7]} "nearmetrics"^{[8]} or inframetrics.^{[9]}
The ρinframetric inequalities were introduced to model roundtrip delay times in the internet.^{[9]} The triangle inequality implies the 2inframetric inequality, and the ultrametric inequality is exactly the 1inframetric inequality.
Premetrics
Relaxing the last three axioms leads to the notion of a premetric, i.e. a function satisfying the following conditions:
 d(x, y) ≥ 0
 d(x, x) = 0
This is not a standard term. Sometimes it is used to refer to other generalizations of metrics such as pseudosemimetrics^{[10]} or pseudometrics;^{[11]} in translations of Russian books it sometimes appears as "prametric".^{[12]}
Any premetric gives rise to a topology as follows. For a positive real r, the "open" rball centred at a point p is defined as
 B_{r}(p) = { x  d(x, p) < r }.
A set is called open if for any point p in the set there is an "open" rball centred at p which is contained in the set. Every premetric space is a topological space, and in fact a sequential space. In general, the "open" rballs themselves need not be open sets with respect to this topology. As for metrics, the distance between two sets A and B, is defined as
 d(A, B) = inf_{x∊A, y∊B} d(x, y).
This defines a premetric on the power set of a premetric space. If we start with a (pseudosemi)metric space, we get a pseudosemimetric, i.e. a symmetric premetric. Any premetric gives rise to a preclosure operator cl as follows:
 cl(A) = { x  d(x, A) = 0 }.
Pseudoquasimetrics
The prefixes pseudo, quasi and semi can also be combined, e.g., a pseudoquasimetric (sometimes called hemimetric) relaxes both the indiscernibility axiom and the symmetry axiom and is simply a premetric satisfying the triangle inequality. For pseudoquasimetric spaces the open rballs form a basis of open sets. A very basic example of a pseudoquasimetric space is the set {0,1} with the premetric given by d(0,1) = 1 and d(1,0) = 0. The associated topological space is the Sierpiński space.
Sets equipped with an extended pseudoquasimetric were studied by William Lawvere as "generalized metric spaces".^{[13]}^{[14]} From a categorical point of view, the extended pseudometric spaces and the extended pseudoquasimetric spaces, along with their corresponding nonexpansive maps, are the best behaved of the metric space categories. One can take arbitrary products and coproducts and form quotient objects within the given category. If one drops "extended", one can only take finite products and coproducts. If one drops "pseudo", one cannot take quotients. Approach spaces are a generalization of metric spaces that maintains these good categorical properties.
Important cases of generalized metrics
In differential geometry, one considers a metric tensor, which can be thought of as an "infinitesimal" quadratic metric function. This is defined as a nondegenerate symmetric bilinear form on the tangent space of a manifold with an appropriate differentiability requirement. While these are not metric functions as defined in this article, they induce what is called a pseudosemimetric function by integration of its square root along a path through the manifold. If one imposes the positivedefiniteness requirement of an inner product on the metric tensor, this restricts to the case of a Riemannian manifold, and the path integration yields a metric.
In general relativity the related concept is a metric tensor (general relativity) which expresses the structure of a pseudoRiemannian manifold. Though the term "metric" is used in cosmology, the fundamental idea is different because there are nonzero null vectors in the tangent space of these manifolds. This generalized view of "metrics", in which zero distance does not imply identity, has crept into some mathematical writing too:^{[15]}^{[16]}
See also
Notes
 ^ ^{a} ^{b} Vitanyi, Paul M. B. (2011). "Information Distance in Multiples". IEEE Transactions on Information Theory. 57 (4): 2451. arXiv:0905.3347 . doi:10.1109/TIT.2011.2110130.
 ^ Cohen, Andrew R.; Vitanyi, Paul M. B. (2012). "Normalized Compression Distance of Multisets with Applications". IEEE Transactions on Pattern Analysis and Machine Intelligence. 37 (8): 1602–1614. arXiv:1212.5711 . doi:10.1109/TPAMI.2014.2375175.
 ^ E.g. Steen & Seebach (1995).
 ^ Smyth, M. (1987). M.Main; A.Melton; M.Mislove; D.Schmidt, eds. Quasi uniformities: reconciling domains with metric spaces. 3rd Conference on Mathematical Foundations of Programming Language Semantics. SpringerVerlag, Lecture Notes in Computer Science 298. pp. 236–253.
 ^ Rolewicz, Stefan (1987), Functional Analysis and Control Theory: Linear Systems, Springer, ISBN 9027721866, OCLC 13064804 This book calls them "semimetrics". That same term is also frequently used for two other generalizations of metrics.
 ^ Väisälä, Jussi (2005), "Gromov hyperbolic spaces" (PDF), Expositiones Mathematicae, 23 (3): 187–231, doi:10.1016/j.exmath.2005.01.010, MR 2164775
 ^ Xia, Q. (2009), "The Geodesic Problem in Quasimetric Spaces", Journal of Geometric Analysis, 19 (2): 452–479, doi:10.1007/s1222000890654
 ^ Qinglan Xia (2008), "The geodesic problem in nearmetric spaces", Journal of Geometric Analysis, 19 (2): 452–479, arXiv:0807.3377 , Bibcode:2008arXiv0807.3377X.
 ^ ^{a} ^{b} * Fraigniaud, P.; Lebhar, E.; Viennot, L. (2008). "The Inframetric Model for the Internet". 2008 IEEE INFOCOM  The 27th Conference on Computer Communications. IEEE INFOCOM 2008. the 27th Conference on Computer Communications. pp. 1085–1093. CiteSeerX 10.1.1.113.6748 . doi:10.1109/INFOCOM.2008.163. ISBN 9781424420261..
 ^ Buldygin, V.V.; Kozachenko, I.U.V. (2000), Metric characterization of random variables and random processes.
 ^ Khelemskiĭ (2006), Lectures and exercises on functional analysis.
 ^ Arkhangel'skii & Pontryagin (1990). Aldrovandi, R.; Pereira, J.G. (1995), An introduction to geometrical physics.
 ^ Lawvere, F.W. (2002) [1973], Metric spaces, generalised logic, and closed categories, Reprints in Theory and Applications of Categories, 1, pp. 1–37.
 ^ Vickers, Steven (2005), "Localic completion of generalized metric spaces I", Theory and Applications of Categories, 14: 328–356
 ^ S. Parrott (1987) Relativistic Electrodynamics and Differential Geometry, page 4, SpringerVerlag ISBN 0387964355 : "This bilinear form is variously called the Lorentz metric, or Minkowski metric or metric tensor."
 ^ Thomas E. Cecil (1992) Lie Sphere Geometry, page 9, SpringerVerlag ISBN 0387977473 : "We call this scalar product the Lorentz metric"
References
 Arkhangel'skii, A. V.; Pontryagin, L. S. (1990), General Topology I: Basic Concepts and Constructions Dimension Theory, Encyclopaedia of Mathematical Sciences, Springer, ISBN 3540181784
 Steen, Lynn Arthur; Seebach, J. Arthur Jr. (1995) [1978], Counterexamples in Topology, Dover, ISBN 9780486687353, MR 0507446, OCLC 32311847
External links
 "Quasimetric space". PlanetMath.
 "Semimetric". PlanetMath.