In mathematical analysis, a metric space M is called **complete** (or a **Cauchy space**) if every Cauchy sequence of points in M has a limit that is also in M.

- Definition
- Examples
- Some theorems
- Completion
- Topologically complete spaces
- Alternatives and generalizations
- See also
- Notes
- References

Intuitively, a space is complete if there are no "points missing" from it (inside or at the boundary). For instance, the set of rational numbers is not complete, because e.g. is "missing" from it, even though one can construct a Cauchy sequence of rational numbers that converges to it (see further examples below). It is always possible to "fill all the holes", leading to the *completion* of a given space, as explained below.

**Definition**: A sequence*x*_{1},*x*_{2},*x*_{3}, ... in a metric space (*X*,*d*) is called**Cauchy**if for every positive real number*r*> 0 there is a positive integer N such that for all positive integers*m*,*n*>*N*,*d*(*x*_{m},*x*_{n}) <*r*.

**Definition**:^{ [1] }The expansion constant of a metric space is the infimum of all constants such that whenever the family intersects pairwise, the intersection is nonempty.

**Definition**: A metric space (*X*,*d*) is**complete**if any of the following equivalent conditions are satisfied:

- Every Cauchy sequence of points in X has a limit that is also in X
- Every Cauchy sequence in X converges in X (that is, to some point of X).
- The expansion constant of (
*X*,*d*) is ≤ 2.^{ [1] } - Every decreasing sequence of non-empty closed subsets of X, with diameters tending to 0, has a non-empty intersection: if
*F*_{n}is closed and non-empty,*F*_{n+1}⊆*F*_{n}for every n, and diam(*F*_{n}) → 0, then there is a point*x*∈*X*common to all sets*F*_{n}.

The space **Q** of rational numbers, with the standard metric given by the absolute value of the difference, is not complete. Consider for instance the sequence defined by *x*_{1} = 1 and This is a Cauchy sequence of rational numbers, but it does not converge towards any rational limit: If the sequence did have a limit x, then by solving necessarily *x*^{2} = 2, yet no rational number has this property. However, considered as a sequence of real numbers, it does converge to the irrational number .

The open interval (0,1), again with the absolute value metric, is not complete either. The sequence defined by *x*_{n} = 1/*n* is Cauchy, but does not have a limit in the given space. However the closed interval [0,1] is complete; for example the given sequence does have a limit in this interval and the limit is zero.

The space **R** of real numbers and the space **C** of complex numbers (with the metric given by the absolute value) are complete, and so is Euclidean space **R**^{n}, with the usual distance metric. In contrast, infinite-dimensional normed vector spaces may or may not be complete; those that are complete are Banach spaces. The space C[*a*, *b*] of continuous real-valued functions on a closed and bounded interval is a Banach space, and so a complete metric space, with respect to the supremum norm. However, the supremum norm does not give a norm on the space C(*a*, *b*) of continuous functions on (*a*, *b*), for it may contain unbounded functions. Instead, with the topology of compact convergence, C(*a*, *b*) can be given the structure of a Fréchet space: a locally convex topological vector space whose topology can be induced by a complete translation-invariant metric.

The space **Q**_{p} of *p*-adic numbers is complete for any prime number p. This space completes **Q** with the *p*-adic metric in the same way that **R** completes **Q** with the usual metric.

If S is an arbitrary set, then the set *S*^{N} of all sequences in S becomes a complete metric space if we define the distance between the sequences (*x*_{n}) and (*y*_{n}) to be 1/*N*, where N is the smallest index for which *x*_{N} is distinct from *y*_{N}, or 0 if there is no such index. This space is homeomorphic to the product of a countable number of copies of the discrete space S.

Riemannian manifolds which are complete are called geodesic manifolds; completeness follows from the Hopf–Rinow theorem.

Every compact metric space is complete, though complete spaces need not be compact. In fact, a metric space is compact if and only if it is complete and totally bounded. This is a generalization of the Heine–Borel theorem, which states that any closed and bounded subspace S of **R**^{n} is compact and therefore complete.^{ [2] }

Let (*X*, *d*) be a complete metric space. If *A* ⊆ *X* is a closed set, then A is also complete.^{ [3] } Let (*X*, *d*) be a metric space. If *A* ⊆ *X* is a complete subspace, then A is also closed.^{ [4] }

If X is a set and M is a complete metric space, then the set B(*X*, *M*) of all bounded functions *f* from X to M is a complete metric space. Here we define the distance in B(*X*, *M*) in terms of the distance in M with the supremum norm

If X is a topological space and M is a complete metric space, then the set *C*_{b}(*X*, *M*) consisting of all continuous bounded functions f from X to M is a closed subspace of B(*X*, *M*) and hence also complete.

The Baire category theorem says that every complete metric space is a Baire space. That is, the union of countably many nowhere dense subsets of the space has empty interior.

The Banach fixed-point theorem states that a contraction mapping on a complete metric space admits a fixed point. The fixed-point theorem is often used to prove the inverse function theorem on complete metric spaces such as Banach spaces.

**Theorem ^{ [5] }** (C. Ursescu) — Let X be a complete metric space and let

- If each
*S*_{i}is closed in X then . - If each
*S*_{i}is open in X then .

For any metric space *M*, one can construct a complete metric space *M′* (which is also denoted as *M*), which contains *M* as a dense subspace. It has the following universal property: if *N* is any complete metric space and *f* is any uniformly continuous function from *M* to *N*, then there exists a unique uniformly continuous function *f′* from *M′* to *N* that extends *f*. The space *M'* is determined up to isometry by this property (among all complete metric spaces isometrically containing *M*), and is called the *completion* of *M*.

The completion of *M* can be constructed as a set of equivalence classes of Cauchy sequences in *M*. For any two Cauchy sequences *x* = (*x*_{n}) and *y* = (*y*_{n}) in *M*, we may define their distance as

(This limit exists because the real numbers are complete.) This is only a pseudometric, not yet a metric, since two different Cauchy sequences may have the distance 0. But "having distance 0" is an equivalence relation on the set of all Cauchy sequences, and the set of equivalence classes is a metric space, the completion of *M*. The original space is embedded in this space via the identification of an element *x* of *M'* with the equivalence class of sequences in *M* converging to *x* (i.e., the equivalence class containing the sequence with constant value *x*). This defines an isometry onto a dense subspace, as required. Notice, however, that this construction makes explicit use of the completeness of the real numbers, so completion of the rational numbers needs a slightly different treatment.

Cantor's construction of the real numbers is similar to the above construction; the real numbers are the completion of the rational numbers using the ordinary absolute value to measure distances. The additional subtlety to contend with is that it is not logically permissible to use the completeness of the real numbers in their own construction. Nevertheless, equivalence classes of Cauchy sequences are defined as above, and the set of equivalence classes is easily shown to be a field that has the rational numbers as a subfield. This field is complete, admits a natural total ordering, and is the unique totally ordered complete field (up to isomorphism). It is *defined* as the field of real numbers (see also Construction of the real numbers for more details). One way to visualize this identification with the real numbers as usually viewed is that the equivalence class consisting of those Cauchy sequences of rational numbers that "ought" to have a given real limit is identified with that real number. The truncations of the decimal expansion give just one choice of Cauchy sequence in the relevant equivalence class.

For a prime *p*, the *p*-adic numbers arise by completing the rational numbers with respect to a different metric.

If the earlier completion procedure is applied to a normed vector space, the result is a Banach space containing the original space as a dense subspace, and if it is applied to an inner product space, the result is a Hilbert space containing the original space as a dense subspace.

Completeness is a property of the *metric* and not of the * topology *, meaning that a complete metric space can be homeomorphic to a non-complete one. An example is given by the real numbers, which are complete but homeomorphic to the open interval (0,1), which is not complete.

In topology one considers * completely metrizable spaces *, spaces for which there exists at least one complete metric inducing the given topology. Completely metrizable spaces can be characterized as those spaces that can be written as an intersection of countably many open subsets of some complete metric space. Since the conclusion of the Baire category theorem is purely topological, it applies to these spaces as well.

Completely metrizable spaces are often called *topologically complete*. However, the latter term is somewhat arbitrary since metric is not the most general structure on a topological space for which one can talk about completeness (see the section Alternatives and generalizations). Indeed, some authors use the term *topologically complete* for a wider class of topological spaces, the completely uniformizable spaces.^{ [6] }

A topological space homeomorphic to a separable complete metric space is called a Polish space.

Since Cauchy sequences can also be defined in general topological groups, an alternative to relying on a metric structure for defining completeness and constructing the completion of a space is to use a group structure. This is most often seen in the context of topological vector spaces, but requires only the existence of a continuous "subtraction" operation. In this setting, the distance between two points *x* and *y* is gauged not by a real number *ε* via the metric *d* in the comparison *d*(*x*, *y*) < *ε*, but by an open neighbourhood *N* of 0 via subtraction in the comparison *x* − *y* ∈ *N*.

A common generalisation of these definitions can be found in the context of a uniform space, where an entourage is a set of all pairs of points that are at no more than a particular "distance" from each other.

It is also possible to replace Cauchy *sequences* in the definition of completeness by Cauchy * nets * or Cauchy filters. If every Cauchy net (or equivalently every Cauchy filter) has a limit in *X*, then *X* is called complete. One can furthermore construct a completion for an arbitrary uniform space similar to the completion of metric spaces. The most general situation in which Cauchy nets apply is Cauchy spaces; these too have a notion of completeness and completion just like uniform spaces.

- Cauchy space
- Completion (algebra)
- Complete uniform space
- Complete topological vector space – A TVS where points that get progressively closer to each other will always converge to a point
- Knaster–Tarski theorem

- 1 2 Grünbaum, B. (1960). "Some applications of expansion constants".
*Pacific J. Math.***10**(1): 193–201. Archived from the original on 2016-03-04. - ↑ Sutherland, Wilson A.
*Introduction to Metric and Topological Spaces*. ISBN 978-0-19-853161-6. - ↑ "Archived copy". Archived from the original on 2007-06-30. Retrieved 2007-01-14.CS1 maint: archived copy as title (link)
- ↑ "Archived copy". Archived from the original on 2007-06-30. Retrieved 2007-01-14.CS1 maint: archived copy as title (link)
- ↑ Zalinescu, C (2002).
*Convex analysis in general vector spaces*. River Edge, N.J. London: World Scientific. p. 33. ISBN 981-238-067-1. OCLC 285163112. - ↑ Kelley, Problem 6.L, p. 208

In mathematics, more specifically in functional analysis, a **Banach space** is a complete normed vector space. Thus, a Banach space is a vector space with a metric that allows the computation of vector length and distance between vectors and is complete in the sense that a Cauchy sequence of vectors always converges to a well defined limit that is within the space.

In mathematics, more specifically in general topology, **compactness** is a property that generalizes the notion of a subset of Euclidean space being closed and bounded. Examples include a closed interval, a rectangle, or a finite set of points. This notion is defined for more general topological spaces than Euclidean space in various ways.

In mathematics, a **Cauchy sequence**, named after Augustin-Louis Cauchy, is a sequence whose elements become arbitrarily close to each other as the sequence progresses. More precisely, given any small positive distance, all but a finite number of elements of the sequence are less than that given distance from each other.

In mathematics, a **continuous function** is a function that does not have any abrupt changes in value, known as discontinuities. More precisely, a function is continuous if arbitrarily small changes in its output can be assured by restricting to sufficiently small changes in its input. If not continuous, a function is said to be *discontinuous*. Up until the 19th century, mathematicians largely relied on intuitive notions of continuity, during which attempts such as the epsilon–delta definition were made to formalize it.

In mathematics, a **metric space** is a set together with a metric on the set. The metric is a function that defines a concept of *distance* between any two members of the set, which are usually called points. The metric satisfies a few simple properties. Informally:

In mathematics, a topological space is called **separable** if it contains a countable, dense subset; that is, there exists a sequence of elements of the space such that every nonempty open subset of the space contains at least one element of the sequence.

In mathematics, a **topological space** is, roughly speaking, a geometrical space in which *closeness* is defined but cannot necessarily be measured by a numeric distance. More specifically, a topological space is a set of points, along with a set of neighbourhoods for each point, satisfying a set of axioms relating points and neighbourhoods.

This is a glossary of some terms used in the branch of mathematics known as topology. Although there is no absolute distinction between different areas of topology, the focus here is on general topology. The following definitions are also fundamental to algebraic topology, differential topology and geometric topology.

In mathematics, **open sets** are a generalization of open intervals in the real line. In a metric space—that is, when a distance is defined—open sets are the sets that, with every point P, contain all points that are sufficiently near to P.

In mathematics, the **closure** of a subset *S* of points in a topological space consists of all points in *S* together with all limit points of *S*. The closure of *S* may equivalently be defined as the union of *S* and its boundary, and also as the intersection of all closed sets containing *S*. Intuitively, the closure can be thought of as all the points that are either in *S* or "near" *S*. A point which is in the closure of *S* is a point of closure of *S*. The notion of closure is in many ways dual to the notion of interior.

In mathematics, a **topological vector space** is one of the basic structures investigated in functional analysis. A topological vector space is a vector space which is also a topological space, this implies that vector space operations be continuous functions. More specifically, its topological space has a uniform topological structure, allowing a notion of uniform convergence.

In geometry, topology, and related branches of mathematics, a **closed set** is a set whose complement is an open set. In a topological space, a closed set can be defined as a set which contains all its limit points. In a complete metric space, a closed set is a set which is closed under the limit operation. This should not be confused with a closed manifold.

In mathematics, a **topological ring** is a ring *R* that is also a topological space such that both the addition and the multiplication are continuous as maps:

In mathematics, **general topology** is the branch of topology that deals with the basic set-theoretic definitions and constructions used in topology. It is the foundation of most other branches of topology, including differential topology, geometric topology, and algebraic topology. Another name for general topology is **point-set topology**.

In functional analysis and related areas of mathematics, **Fréchet spaces**, named after Maurice Fréchet, are special topological vector spaces. They are generalizations of Banach spaces. All Banach and Hilbert spaces are Fréchet spaces. Spaces of infinitely differentiable functions are typical examples of Fréchet spaces, many of which are typically *not* Banach spaces.

In functional analysis and related areas of mathematics, **locally convex topological vector spaces** (**LCTVS**) or **locally convex spaces** are examples of topological vector spaces (TVS) that generalize normed spaces. They can be defined as topological vector spaces whose topology is generated by translations of balanced, absorbent, convex sets. Alternatively they can be defined as a vector space with a family of seminorms, and a topology can be defined in terms of that family. Although in general such spaces are not necessarily normable, the existence of a convex local base for the zero vector is strong enough for the Hahn–Banach theorem to hold, yielding a sufficiently rich theory of continuous linear functionals.

In topology and related branches of mathematics, **total-boundedness** is a generalization of compactness for circumstances in which a set is not necessarily closed. A totally bounded set can be covered by finitely many subsets of every fixed "size"

In functional analysis and related areas of mathematics, a **sequence space** is a vector space whose elements are infinite sequences of real or complex numbers. Equivalently, it is a function space whose elements are functions from the natural numbers to the field *K* of real or complex numbers. The set of all such functions is naturally identified with the set of all possible infinite sequences with elements in *K*, and can be turned into a vector space under the operations of pointwise addition of functions and pointwise scalar multiplication. All sequence spaces are linear subspaces of this space. Sequence spaces are typically equipped with a norm, or at least the structure of a topological vector space.

In functional analysis and related areas of mathematics, a **complete topological vector space** is a topological vector space (TVS) with the property that whenever points get progressively closer to each other, then there exists some point towards which they all get closer to. The notion of "points that get progressively closer" is made rigorous by *Cauchy nets* or *Cauchy filters*, which are generalizations of *Cauchy sequences*, while "point towards which they all get closer to" means that this net or filter converges to Unlike the notion of completeness for metric spaces, which it generalizes, the notion of completeness for TVSs does not depend on any metric and is defined for *all* TVSs, including those that are not metrizable or Hausdorff.

This is a glossary for the terminology in a mathematical field of functional analysis.

- Kelley, John L. (1975).
*General Topology*. Springer. ISBN 0-387-90125-6. - Kreyszig, Erwin,
*Introductory functional analysis with applications*(Wiley, New York, 1978). ISBN 0-471-03729-X - Lang, Serge, "Real and Functional Analysis" ISBN 0-387-94001-4
- Meise, Reinhold; Vogt, Dietmar (1997).
*Introduction to functional analysis*. Ramanujan, M.S. (trans.). Oxford: Clarendon Press; New York: Oxford University Press. ISBN 0-19-851485-9.

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