Vector space

A vector space (also called a linear space) is a set of objects called "vectors", which may be added together and multiplied ("scaled") by numbers, called "scalars". Scalars are often taken to be real numbers, but there are also vector spaces with scalar multiplication by complex numbers, rational numbers, or generally any field. The operations of vector addition and scalar multiplication must satisfy certain requirements, called vector "axioms" (listed below in ). To specify that the scalars are real or complex numbers, the terms real vector space and complex vector space are often used.
Certain sets of Euclidean vectors are common examples of a vector space. They represent physical quantities such as forces, where any two forces (of the same type) can be added to yield a third, and the multiplication of a force vector by a real multiplier is another force vector. In the same way (but in a more geometric sense), vectors representing displacements in the plane or three-dimensional space also form vector spaces. Vectors in vector spaces do not necessarily have to be arrow-like objects as they appear in the mentioned examples: vectors are regarded as abstract mathematical objects with particular properties, which in some cases can be visualized as arrows.
Vector spaces are the subject of linear algebra and are well characterized by their dimension, which, roughly speaking, specifies the number of independent directions in the space. Infinite-dimensional vector spaces arise naturally in mathematical analysis as function spaces, whose vectors are functions. These vector spaces are generally endowed with some additional structure such as a topology, which allows the consideration of issues of proximity and continuity. Among these topologies, those that are defined by a norm or inner product are more commonly used (being equipped with a notion of distance between two vectors). This is particularly the case of Banach spaces and Hilbert spaces, which are fundamental in mathematical analysis.
Historically, the first ideas leading to vector spaces can be traced back as far as the 17th century's analytic geometry, matrices, systems of linear equations and Euclidean vectors. The modern, more abstract treatment, first formulated by Giuseppe Peano in 1888, encompasses more general objects than Euclidean space, but much of the theory can be seen as an extension of classical geometric ideas like lines, planes and their higher-dimensional analogs.
Today, vector spaces are applied throughout mathematics, science and engineering. They are the appropriate linear-algebraic notion to deal with systems of linear equations. They offer a framework for Fourier expansion, which is employed in image compression routines, and they provide an environment that can be used for solution techniques for partial differential equations. Furthermore, vector spaces furnish an abstract, coordinate-free way of dealing with geometrical and physical objects such as tensors. This in turn allows the examination of local properties of manifolds by linearization techniques. Vector spaces may be generalized in several ways, leading to more advanced notions in geometry and abstract algebra.
This article deals mainly with finite-dimensional vector spaces. However, many of the principles are also valid for infinite-dimensional vector spaces.
Introduction and definition.
The concept of vector space will first be explained by describing two particular examples:
First example: arrows in the plane.
The first example of a vector space consists of arrows in a fixed plane, starting at one fixed point. This is used in physics to describe forces or velocities. Given any two such arrows, and , the parallelogram spanned by these two arrows contains one diagonal arrow that starts at the origin, too. This new arrow is called the "sum" of the two arrows, and is denoted . In the special case of two arrows on the same line, their sum is the arrow on this line whose length is the sum or the difference of the lengths, depending on whether the arrows have the same direction. Another operation that can be done with arrows is scaling: given any positive real number , the arrow that has the same direction as , but is dilated or shrunk by multiplying its length by , is called "multiplication" of by . It is denoted . When is negative, is defined as the arrow pointing in the opposite direction instead.
The following shows a few examples: if , the resulting vector has the same direction as , but is stretched to the double length of (right image below). Equivalently, is the sum . Moreover, has the opposite direction and the same length as (blue vector pointing down in the right image).
Second example: ordered pairs of numbers.
A second key example of a vector space is provided by pairs of real numbers and . (The order of the components and is significant, so such a pair is also called an ordered pair.) Such a pair is written as . The sum of two such pairs and multiplication of a pair with a number is defined as follows:
and
The first example above reduces to this one, if the arrows are represented by the pair of Cartesian coordinates of their endpoints.
Definition.
In this article, vectors are represented in boldface to distinguish them from scalars.
A vector space over a field is a set  together with two operations that satisfy the eight axioms listed below. In the following, denotes the Cartesian product of with itself, and denotes a mapping from one set to another.
Elements of are commonly called "vectors". Elements of  are commonly called "scalars". Common symbols for denoting vector spaces include , , and .
In the two examples above, the field is the field of the real numbers, and the set of the vectors consists of the planar arrows with fixed starting point and pairs of real numbers, respectively.
To qualify as a vector space, the set  and the operations of addition and multiplication must adhere to a number of requirements called axioms. These are listed in the table below, where , and denote arbitrary vectors in , and and denote scalars in .
These axioms generalize properties of the vectors introduced in the above examples. Indeed, the result of addition of two ordered pairs (as in the second example above) does not depend on the order of the summands:
Likewise, in the geometric example of vectors as arrows, since the parallelogram defining the sum of the vectors is independent of the order of the vectors. All other axioms can be verified in a similar manner in both examples. Thus, by disregarding the concrete nature of the particular type of vectors, the definition incorporates these two and many more examples in one notion of vector space.
Subtraction of two vectors and division by a (non-zero) scalar can be defined as
When the scalar field is the real numbers , the vector space is called a "real vector space". When the scalar field is the complex numbers , the vector space is called a "complex vector space". These two cases are the ones used most often in engineering. The general definition of a vector space allows scalars to be elements of any fixed field . The notion is then known as an -"vector space" or a "vector space over ". A field is, essentially, a set of numbers possessing addition, subtraction, multiplication and division operations. For example, rational numbers form a field.
In contrast to the intuition stemming from vectors in the plane and higher-dimensional cases, in general vector spaces, there is no notion of nearness, angles or distances. To deal with such matters, particular types of vector spaces are introduced; see § Vector spaces with additional structure below for more.
Alternative formulations and elementary consequences.
Vector addition and scalar multiplication are operations, satisfying the closure property: and are in for all in , and , in . Some older sources mention these properties as separate axioms.
In the parlance of abstract algebra, the first four axioms are equivalent to requiring the set of vectors to be an abelian group under addition. The remaining axioms give this group an -module structure. In other words, there is a ring homomorphism from the field into the endomorphism ring of the group of vectors. Then scalar multiplication is defined as .
There are a number of direct consequences of the vector space axioms. Some of them derive from elementary group theory, applied to the additive group of vectors: for example, the zero vector of and the additive inverse of any vector are unique. Further properties follow by employing also the distributive law for the scalar multiplication, for example equals if and only if equals or equals .
History.
Vector spaces stem from affine geometry, via the introduction of coordinates in the plane or three-dimensional space. Around 1636, French mathematicians René Descartes and Pierre de Fermat founded analytic geometry by identifying solutions to an equation of two variables with points on a plane curve. To achieve geometric solutions without using coordinates, Bolzano introduced, in 1804, certain operations on points, lines and planes, which are predecessors of vectors. introduced the notion of barycentric coordinates. introduced the notion of a bipoint, i.e., an oriented segment one of whose ends is the origin and the other one a target. Vectors were reconsidered with the presentation of complex numbers by Argand and Hamilton and the inception of quaternions by the latter. They are elements in R2 and R4; treating them using linear combinations goes back to Laguerre in 1867, who also defined systems of linear equations.
In 1857, Cayley introduced the matrix notation which allows for a harmonization and simplification of linear maps. Around the same time, Grassmann studied the barycentric calculus initiated by Möbius. He envisaged sets of abstract objects endowed with operations. In his work, the concepts of linear independence and dimension, as well as scalar products are present. Actually Grassmann's 1844 work exceeds the framework of vector spaces, since his considering multiplication, too, led him to what are today called algebras. Italian mathematician Peano was the first to give the modern definition of vector spaces and linear maps in 1888.
An important development of vector spaces is due to the construction of function spaces by Henri Lebesgue. This was later formalized by Banach and Hilbert, around 1920. At that time, algebra and the new field of functional analysis began to interact, notably with key concepts such as spaces of "p"-integrable functions and Hilbert spaces. Also at this time, the first studies concerning infinite-dimensional vector spaces were done.
Examples.
Coordinate space.
The simplest example of a vector space over a field is the field itself, equipped with its standard addition and multiplication. More generally, all -tuples (sequences of length ) 
of elements of form a vector space that is usually denoted and called a coordinate space. 
The case is the above-mentioned simplest example, in which the field is also regarded as a vector space over itself. The case and was discussed in the introduction above.
Complex numbers and other field extensions.
The set of complex numbers , that is, numbers that can be written in the form for real numbers and where is the imaginary unit, form a vector space over the reals with the usual addition and multiplication: and for real numbers , , , and . The various axioms of a vector space follow from the fact that the same rules hold for complex number arithmetic.
In fact, the example of complex numbers is essentially the same as (that is, it is "isomorphic" to) the vector space of ordered pairs of real numbers mentioned above: if we think of the complex number as representing the ordered pair in the complex plane then we see that the rules for addition and scalar multiplication correspond exactly to those in the earlier example.
More generally, field extensions provide another class of examples of vector spaces, particularly in algebra and algebraic number theory: a field containing a smaller field is an -vector space, by the given multiplication and addition operations of . For example, the complex numbers are a vector space over , and the field extension formula_4 is a vector space over . 
Function spaces.
Functions from any fixed set to a field also form vector spaces, by performing addition and scalar multiplication pointwise. That is, the sum of two functions and is the function given by
and similarly for multiplication. Such function spaces occur in many geometric situations, when is the real line or an interval, or other subsets of . Many notions in topology and analysis, such as continuity, integrability or differentiability are well-behaved with respect to linearity: sums and scalar multiples of functions possessing such a property still have that property. Therefore, the set of such functions are vector spaces. They are studied in greater detail using the methods of functional analysis, see below. Algebraic constraints also yield vector spaces: the vector space is given by polynomial functions:
Linear equations.
Systems of homogeneous linear equations are closely tied to vector spaces. For example, the solutions of 
are given by triples with arbitrary , , and . They form a vector space: sums and scalar multiples of such triples still satisfy the same ratios of the three variables; thus they are solutions, too. Matrices can be used to condense multiple linear equations as above into one vector equation, namely
where formula_5 is the matrix containing the coefficients of the given equations, is the vector , denotes the matrix product, and is the zero vector. In a similar vein, the solutions of homogeneous "linear differential equations" form vector spaces. For example,
yields , where and are arbitrary constants, and is the natural exponential function.
Basis and dimension.
"Bases" allow one to represent vectors by a sequence of scalars called "coordinates" or "components". A basis is a set of vectors , for convenience often indexed by some index set , that spans the whole space and is linearly independent. "Spanning the whole space" means that any vector can be expressed as a finite sum (called a "linear combination") of the basis elements:
where the are scalars, called the coordinates (or the components) of the vector with respect to the basis , and elements of . Linear independence means that the coordinates are uniquely determined for any vector in the vector space.
For example, the coordinate vectors , , to , form a basis of , called the standard basis, since any vector can be uniquely expressed as a linear combination of these vectors:
The corresponding coordinates , , , are just the Cartesian coordinates of the vector.
Every vector space has a basis. This follows from Zorn's lemma, an equivalent formulation of the Axiom of Choice. Given the other axioms of Zermelo–Fraenkel set theory, the existence of bases is equivalent to the axiom of choice. The ultrafilter lemma, which is weaker than the axiom of choice, implies that all bases of a given vector space have the same number of elements, or cardinality (cf. "Dimension theorem for vector spaces"). It is called the "dimension" of the vector space, denoted by dim "V". If the space is spanned by finitely many vectors, the above statements can be proven without such fundamental input from set theory.
The dimension of the coordinate space is , by the basis exhibited above. The dimension of the polynomial ring "F"["x"] introduced above is countably infinite, a basis is given by , , , A fortiori, the dimension of more general function spaces, such as the space of functions on some (bounded or unbounded) interval, is infinite. Under suitable regularity assumptions on the coefficients involved, the dimension of the solution space of a homogeneous ordinary differential equation equals the degree of the equation. For example, the solution space for the above equation is generated by . These two functions are linearly independent over , so the dimension of this space is two, as is the degree of the equation.
A field extension over the rationals can be thought of as a vector space over (by defining vector addition as field addition, defining scalar multiplication as field multiplication by elements of , and otherwise ignoring the field multiplication). The dimension (or degree) of the field extension over depends on . If satisfies some polynomial equation
formula_6
with rational coefficients (in other words, if "α" is algebraic), the dimension is finite. More precisely, it equals the degree of the minimal polynomial having α as a root. For example, the complex numbers C are a two-dimensional real vector space, generated by 1 and the imaginary unit "i". The latter satisfies "i"2 + 1 = 0, an equation of degree two. Thus, C is a two-dimensional R-vector space (and, as any field, one-dimensional as a vector space over itself, C). If "α" is not algebraic, the dimension of Q("α") over Q is infinite. For example, for "α" = π there is no such equation. That is, π is transcendental.
Linear maps and matrices.
The relation of two vector spaces can be expressed by "linear map" or "linear transformation". They are functions that reflect the vector space structure, that is, they preserve sums and scalar multiplication:
An "isomorphism" is a linear map such that there exists an inverse map , which is a map such that the two possible compositions and are identity maps. Equivalently, is both one-to-one (injective) and onto (surjective). If there exists an isomorphism between and , the two spaces are said to be "isomorphic"; they are then essentially identical as vector spaces, since all identities holding in are, via , transported to similar ones in , and vice versa via .
For example, the "arrows in the plane" and "ordered pairs of numbers" vector spaces in the introduction are isomorphic: a planar arrow departing at the origin of some (fixed) coordinate system can be expressed as an ordered pair by considering the - and -component of the arrow, as shown in the image at the right. Conversely, given a pair , the arrow going by to the right (or to the left, if is negative), and up (down, if is negative) turns back the arrow .
Linear maps between two vector spaces form a vector space , also denoted , or . The space of linear maps from to is called the "dual vector space", denoted . Via the injective natural map , any vector space can be embedded into its "bidual"; the map is an isomorphism if and only if the space is finite-dimensional.
Once a basis of is chosen, linear maps are completely determined by specifying the images of the basis vectors, because any element of is expressed uniquely as a linear combination of them. If , a 1-to-1 correspondence between fixed bases of and gives rise to a linear map that maps any basis element of to the corresponding basis element of . It is an isomorphism, by its very definition. Therefore, two vector spaces are isomorphic if their dimensions agree and vice versa. Another way to express this is that any vector space is "completely classified" (up to isomorphism) by its dimension, a single number. In particular, any "n"-dimensional -vector space is isomorphic to . There is, however, no "canonical" or preferred isomorphism; actually an isomorphism is equivalent to the choice of a basis of , by mapping the standard basis of to , via . The freedom of choosing a convenient basis is particularly useful in the infinite-dimensional context; see below.
Matrices.
"Matrices" are a useful notion to encode linear maps. They are written as a rectangular array of scalars as in the image at the right. Any -by- matrix gives rise to a linear map from to , by the following
or, using the matrix multiplication of the matrix with the coordinate vector :
Moreover, after choosing bases of and , "any" linear map is uniquely represented by a matrix via this assignment.
The determinant of a square matrix is a scalar that tells whether the associated map is an isomorphism or not: to be so it is sufficient and necessary that the determinant is nonzero. The linear transformation of corresponding to a real "n"-by-"n" matrix is orientation preserving if and only if its determinant is positive.
Eigenvalues and eigenvectors.
Endomorphisms, linear maps , are particularly important since in this case vectors can be compared with their image under , . Any nonzero vector satisfying , where is a scalar, is called an "eigenvector" of with "eigenvalue" . Equivalently, is an element of the kernel of the difference (where Id is the identity map . If is finite-dimensional, this can be rephrased using determinants: having eigenvalue is equivalent to
By spelling out the definition of the determinant, the expression on the left hand side can be seen to be a polynomial function in , called the characteristic polynomial of . If the field is large enough to contain a zero of this polynomial (which automatically happens for algebraically closed, such as ) any linear map has at least one eigenvector. The vector space may or may not possess an eigenbasis, a basis consisting of eigenvectors. This phenomenon is governed by the Jordan canonical form of the map. The set of all eigenvectors corresponding to a particular eigenvalue of forms a vector space known as the "eigenspace" corresponding to the eigenvalue (and ) in question. To achieve the spectral theorem, the corresponding statement in the infinite-dimensional case, the machinery of functional analysis is needed, see below.
Basic constructions.
In addition to the above concrete examples, there are a number of standard linear algebraic constructions that yield vector spaces related to given ones. In addition to the definitions given below, they are also characterized by universal properties, which determine an object by specifying the linear maps from to any other vector space.
Subspaces and quotient spaces.
A nonempty subset "W" of a vector space "V" that is closed under addition and scalar multiplication (and therefore contains the 0-vector of "V") is called a "linear subspace" of "V", or simply a "subspace" of "V", when the ambient space is unambiguously a vector space. Subspaces of "V" are vector spaces (over the same field) in their own right. The intersection of all subspaces containing a given set "S" of vectors is called its span, and it is the smallest subspace of "V" containing the set "S". Expressed in terms of elements, the span is the subspace consisting of all the linear combinations of elements of "S".
A linear subspace of dimension 1 is a vector line. A linear subspace of dimension 2 is a vector plane. A linear subspace that contains all elements but one of a basis of the ambient space is a vector hyperplane. In a vector space of finite dimension , a vector hyperplane is thus a subspace of dimension .
The counterpart to subspaces are "quotient vector spaces". Given any subspace , the quotient space "V"/"W" (""V" modulo "W"") is defined as follows: as a set, it consists of where v is an arbitrary vector in "V". The sum of two such elements and is and scalar multiplication is given by . The key point in this definition is that if and only if the difference of v1 and v2 lies in "W". This way, the quotient space "forgets" information that is contained in the subspace "W".
The kernel ker("f") of a linear map consists of vectors v that are mapped to 0 in "W". The kernel and the image are subspaces of "V" and "W", respectively. The existence of kernels and images is part of the statement that the category of vector spaces (over a fixed field "F") is an abelian category, that is, a corpus of mathematical objects and structure-preserving maps between them (a category) that behaves much like the category of abelian groups. Because of this, many statements such as the first isomorphism theorem (also called rank–nullity theorem in matrix-related terms)
and the second and third isomorphism theorem can be formulated and proven in a way very similar to the corresponding statements for groups.
An important example is the kernel of a linear map for some fixed matrix "A", as above. The kernel of this map is the subspace of vectors x such that , which is precisely the set of solutions to the system of homogeneous linear equations belonging to "A". This concept also extends to linear differential equations
In the corresponding map
the derivatives of the function "f" appear linearly (as opposed to "f"′′("x")2, for example). Since differentiation is a linear procedure (that is, and for a constant ) this assignment is linear, called a linear differential operator. In particular, the solutions to the differential equation form a vector space (over or ).
Direct product and direct sum.
The "direct product" of vector spaces and the "direct sum" of vector spaces are two ways of combining an indexed family of vector spaces into a new vector space.
The "direct product" formula_12 of a family of vector spaces "V""i" consists of the set of all tuples (, which specify for each index "i" in some index set "I" an element v"i" of "V""i". Addition and scalar multiplication is performed componentwise. A variant of this construction is the "direct sum" formula_13 (also called coproduct and denoted formula_14), where only tuples with finitely many nonzero vectors are allowed. If the index set "I" is finite, the two constructions agree, but in general they are different.
Tensor product.
The "tensor product" , or simply , of two vector spaces "V" and "W" is one of the central notions of multilinear algebra which deals with extending notions such as linear maps to several variables. A map is called bilinear if "g" is linear in both variables v and w. That is to say, for fixed w the map is linear in the sense above and likewise for fixed v.
The tensor product is a particular vector space that is a "universal" recipient of bilinear maps "g", as follows. It is defined as the vector space consisting of finite (formal) sums of symbols called tensors
subject to the rules 
These rules ensure that the map "f" from the to that maps a tuple to is bilinear. The universality states that given "any" vector space "X" and "any" bilinear map , there exists a unique map "u", shown in the diagram with a dotted arrow, whose composition with "f" equals "g": . This is called the universal property of the tensor product, an instance of the method—much used in advanced abstract algebra—to indirectly define objects by specifying maps from or to this object.
Vector spaces with additional structure.
From the point of view of linear algebra, vector spaces are completely understood insofar as any vector space is characterized, up to isomorphism, by its dimension. However, vector spaces "per se" do not offer a framework to deal with the question—crucial to analysis—whether a sequence of functions converges to another function. Likewise, linear algebra is not adapted to deal with infinite series, since the addition operation allows only finitely many terms to be added. Therefore, the needs of functional analysis require considering additional structures.
A vector space may be given a partial order ≤, under which some vectors can be compared. For example, "n"-dimensional real space R"n" can be ordered by comparing its vectors componentwise. Ordered vector spaces, for example Riesz spaces, are fundamental to Lebesgue integration, which relies on the ability to express a function as a difference of two positive functions
where "f"+ denotes the positive part of "f" and "f"− the negative part.
Normed vector spaces and inner product spaces.
"Measuring" vectors is done by specifying a norm, a datum which measures lengths of vectors, or by an inner product, which measures angles between vectors. Norms and inner products are denoted formula_15 and respectively. The datum of an inner product entails that lengths of vectors can be defined too, by defining the associated norm Vector spaces endowed with such data are known as "normed vector spaces" and "inner product spaces", respectively.
Coordinate space "F""n" can be equipped with the standard dot product:
In R2, this reflects the common notion of the angle between two vectors x and y, by the law of cosines:
Because of this, two vectors satisfying formula_18 are called orthogonal. An important variant of the standard dot product is used in Minkowski space: R4 endowed with the Lorentz product
In contrast to the standard dot product, it is not positive definite: formula_20 also takes negative values, for example, for formula_21. Singling out the fourth coordinate—corresponding to time, as opposed to three space-dimensions—makes it useful for the mathematical treatment of special relativity.
Topological vector spaces.
Convergence questions are treated by considering vector spaces "V" carrying a compatible topology, a structure that allows one to talk about elements being close to each other. Compatible here means that addition and scalar multiplication have to be continuous maps. Roughly, if x and y in "V", and "a" in "F" vary by a bounded amount, then so do and . To make sense of specifying the amount a scalar changes, the field "F" also has to carry a topology in this context; a common choice are the reals or the complex numbers.
In such "topological vector spaces" one can consider series of vectors. The infinite sum
denotes the limit of the corresponding finite partial sums of the sequence ("f""i")"i"∈N of elements of "V". For example, the "f""i" could be (real or complex) functions belonging to some function space "V", in which case the series is a function series. The mode of convergence of the series depends on the topology imposed on the function space. In such cases, pointwise convergence and uniform convergence are two prominent examples.
A way to ensure the existence of limits of certain infinite series is to restrict attention to spaces where any Cauchy sequence has a limit; such a vector space is called complete. Roughly, a vector space is complete provided that it contains all necessary limits. For example, the vector space of polynomials on the unit interval [0,1], equipped with the topology of uniform convergence is not complete because any continuous function on [0,1] can be uniformly approximated by a sequence of polynomials, by the Weierstrass approximation theorem. In contrast, the space of "all" continuous functions on [0,1] with the same topology is complete. A norm gives rise to a topology by defining that a sequence of vectors v"n" converges to v if and only if
Banach and Hilbert spaces are complete topological vector spaces whose topologies are given, respectively, by a norm and an inner product. Their study—a key piece of functional analysis—focuses on infinite-dimensional vector spaces, since all norms on finite-dimensional topological vector spaces give rise to the same notion of convergence. The image at the right shows the equivalence of the 1-norm and ∞-norm on R2: as the unit "balls" enclose each other, a sequence converges to zero in one norm if and only if it so does in the other norm. In the infinite-dimensional case, however, there will generally be inequivalent topologies, which makes the study of topological vector spaces richer than that of vector spaces without additional data.
From a conceptual point of view, all notions related to topological vector spaces should match the topology. For example, instead of considering all linear maps (also called functionals) , maps between topological vector spaces are required to be continuous. In particular, the (topological) dual space consists of continuous functionals (or to ). The fundamental Hahn–Banach theorem is concerned with separating subspaces of appropriate topological vector spaces by continuous functionals.
Banach spaces.
"Banach spaces", introduced by Stefan Banach, are complete normed vector spaces.
A first example is the vector space formula_24 consisting of infinite vectors with real entries 
formula_25
whose formula_26-norm 
formula_27
given by 
The topologies on the infinite-dimensional space formula_24 
are inequivalent for different formula_26. For example, the sequence of vectors 
formula_33, 
in which the first formula_34 components are formula_35 and the following ones are formula_36, converges to the zero vector for 
formula_37, 
but does not for 
formula_38:
More generally than sequences of real numbers, functions 
formula_41
are endowed with a norm that replaces the above sum by the Lebesgue integral
The space of integrable functions on a given domain 
formula_43 (for example an interval) satisfying 
formula_44,
and equipped with this norm are called Lebesgue spaces, denoted formula_45.
These spaces are complete. (If one uses the Riemann integral instead, the space is "not" complete, which may be seen as a justification for Lebesgue's integration theory.) Concretely this means that for any sequence of Lebesgue-integrable functions   
formula_46  
with
formula_47, 
satisfying the condition
there exists a function formula_49 belonging to the vector space formula_45 such that
Imposing boundedness conditions not only on the function, but also on its derivatives leads to Sobolev spaces.
Hilbert spaces.
Complete inner product spaces are known as "Hilbert spaces", in honor of David Hilbert. The Hilbert space "L"2(Ω), with inner product given by
where formula_53 denotes the complex conjugate of "g"("x"), is a key case.
By definition, in a Hilbert space any Cauchy sequence converges to a limit. Conversely, finding a sequence of functions "f""n" with desirable properties that approximates a given limit function, is equally crucial. Early analysis, in the guise of the Taylor approximation, established an approximation of differentiable functions "f" by polynomials. By the Stone–Weierstrass theorem, every continuous function on can be approximated as closely as desired by a polynomial. A similar approximation technique by trigonometric functions is commonly called Fourier expansion, and is much applied in engineering, see below. More generally, and more conceptually, the theorem yields a simple description of what "basic functions", or, in abstract Hilbert spaces, what basic vectors suffice to generate a Hilbert space "H", in the sense that the "closure" of their span (that is, finite linear combinations and limits of those) is the whole space. Such a set of functions is called a "basis" of "H", its cardinality is known as the Hilbert space dimension. Not only does the theorem exhibit suitable basis functions as sufficient for approximation purposes, but also together with the Gram–Schmidt process, it enables one to construct a basis of orthogonal vectors. Such orthogonal bases are the Hilbert space generalization of the coordinate axes in finite-dimensional Euclidean space.
The solutions to various differential equations can be interpreted in terms of Hilbert spaces. For example, a great many fields in physics and engineering lead to such equations and frequently solutions with particular physical properties are used as basis functions, often orthogonal. As an example from physics, the time-dependent Schrödinger equation in quantum mechanics describes the change of physical properties in time by means of a partial differential equation, whose solutions are called wavefunctions. Definite values for physical properties such as energy, or momentum, correspond to eigenvalues of a certain (linear) differential operator and the associated wavefunctions are called eigenstates. The spectral theorem decomposes a linear compact operator acting on functions in terms of these eigenfunctions and their eigenvalues.
Algebras over fields.
General vector spaces do not possess a multiplication between vectors. A vector space equipped with an additional bilinear operator defining the multiplication of two vectors is an "algebra over a field". Many algebras stem from functions on some geometrical object: since functions with values in a given field can be multiplied pointwise, these entities form algebras. The Stone–Weierstrass theorem, for example, relies on Banach algebras which are both Banach spaces and algebras.
Commutative algebra makes great use of rings of polynomials in one or several variables, introduced above. Their multiplication is both commutative and associative. These rings and their quotients form the basis of algebraic geometry, because they are rings of functions of algebraic geometric objects.
Another crucial example are "Lie algebras", which are neither commutative nor associative, but the failure to be so is limited by the constraints ( denotes the product of and ):
Examples include the vector space of "n"-by-"n" matrices, with , the commutator of two matrices, and , endowed with the cross product.
The tensor algebra T("V") is a formal way of adding products to any vector space "V" to obtain an algebra. As a vector space, it is spanned by symbols, called simple tensors
The multiplication is given by concatenating such symbols, imposing the distributive law under addition, and requiring that scalar multiplication commute with the tensor product ⊗, much the same way as with the tensor product of two vector spaces introduced above. In general, there are no relations between and . Forcing two such elements to be equal leads to the symmetric algebra, whereas forcing yields the exterior algebra.
When a field, is explicitly stated, a common term used is -algebra.
Applications.
Vector spaces have many applications as they occur frequently in common circumstances, namely wherever functions with values in some field are involved. They provide a framework to deal with analytical and geometrical problems, or are used in the Fourier transform. This list is not exhaustive: many more applications exist, for example in optimization. The minimax theorem of game theory stating the existence of a unique payoff when all players play optimally can be formulated and proven using vector spaces methods. Representation theory fruitfully transfers the good understanding of linear algebra and vector spaces to other mathematical domains such as group theory.
Distributions.
A "distribution" (or "generalized function") is a linear map assigning a number to each "test" function, typically a smooth function with compact support, in a continuous way: in the above terminology the space of distributions is the (continuous) dual of the test function space. The latter space is endowed with a topology that takes into account not only "f" itself, but also all its higher derivatives. A standard example is the result of integrating a test function "f" over some domain Ω:
When , the set consisting of a single point, this reduces to the Dirac distribution, denoted by "δ", which associates to a test function "f" its value at the . Distributions are a powerful instrument to solve differential equations. Since all standard analytic notions such as derivatives are linear, they extend naturally to the space of distributions. Therefore, the equation in question can be transferred to a distribution space, which is bigger than the underlying function space, so that more flexible methods are available for solving the equation. For example, Green's functions and fundamental solutions are usually distributions rather than proper functions, and can then be used to find solutions of the equation with prescribed boundary conditions. The found solution can then in some cases be proven to be actually a true function, and a solution to the original equation (for example, using the Lax–Milgram theorem, a consequence of the Riesz representation theorem).
Fourier analysis.
Resolving a periodic function into a sum of trigonometric functions forms a "Fourier series", a technique much used in physics and engineering. The underlying vector space is usually the Hilbert space "L"2(0, 2π), for which the functions sin("mx") and cos("mx") (where "m" is an integer) form an orthogonal basis. The Fourier expansion of an "L"2 function "f" is
The coefficients "a""m" and "b""m" are called Fourier coefficients of "f", and are calculated by the formulas
In physical terms the function is represented as a superposition of sine waves and the coefficients give information about the function's frequency spectrum. A complex-number form of Fourier series is also commonly used. The concrete formulae above are consequences of a more general mathematical duality called Pontryagin duality. Applied to the group R, it yields the classical Fourier transform; an application in physics are reciprocal lattices, where the underlying group is a finite-dimensional real vector space endowed with the additional datum of a lattice encoding positions of atoms in crystals.
Fourier series are used to solve boundary value problems in partial differential equations. In 1822, Fourier first used this technique to solve the heat equation. A discrete version of the Fourier series can be used in sampling applications where the function value is known only at a finite number of equally spaced points. In this case the Fourier series is finite and its value is equal to the sampled values at all points. The set of coefficients is known as the discrete Fourier transform (DFT) of the given sample sequence. The DFT is one of the key tools of digital signal processing, a field whose applications include radar, speech encoding, image compression. The JPEG image format is an application of the closely related discrete cosine transform.
The fast Fourier transform is an algorithm for rapidly computing the discrete Fourier transform. It is used not only for calculating the Fourier coefficients but, using the convolution theorem, also for computing the convolution of two finite sequences. They in turn are applied in digital filters and as a rapid multiplication algorithm for polynomials and large integers (Schönhage–Strassen algorithm).
Differential geometry.
The tangent plane to a surface at a point is naturally a vector space whose origin is identified with the point of contact. The tangent plane is the best linear approximation, or linearization, of a surface at a point. Even in a three-dimensional Euclidean space, there is typically no natural way to prescribe a basis of the tangent plane, and so it is conceived of as an abstract vector space rather than a real coordinate space. The "tangent space" is the generalization to higher-dimensional differentiable manifolds.
Riemannian manifolds are manifolds whose tangent spaces are endowed with a suitable inner product. Derived therefrom, the Riemann curvature tensor encodes all curvatures of a manifold in one object, which finds applications in general relativity, for example, where the Einstein curvature tensor describes the matter and energy content of space-time. The tangent space of a Lie group can be given naturally the structure of a Lie algebra and can be used to classify compact Lie groups.
Generalizations.
Vector bundles.
A "vector bundle" is a family of vector spaces parametrized continuously by a topological space "X". More precisely, a vector bundle over "X" is a topological space "E" equipped with a continuous map 
such that for every "x" in "X", the fiber π−1("x") is a vector space. The case dim is called a line bundle. For any vector space "V", the projection makes the product into a "trivial" vector bundle. Vector bundles over "X" are required to be locally a product of "X" and some (fixed) vector space "V": for every "x" in "X", there is a neighborhood "U" of "x" such that the restriction of π to π−1("U") is isomorphic to the trivial bundle . Despite their locally trivial character, vector bundles may (depending on the shape of the underlying space "X") be "twisted" in the large (that is, the bundle need not be (globally isomorphic to) the trivial bundle ). For example, the Möbius strip can be seen as a line bundle over the circle "S"1 (by identifying open intervals with the real line). It is, however, different from the cylinder , because the latter is orientable whereas the former is not.
Properties of certain vector bundles provide information about the underlying topological space. For example, the tangent bundle consists of the collection of tangent spaces parametrized by the points of a differentiable manifold. The tangent bundle of the circle "S"1 is globally isomorphic to , since there is a global nonzero vector field on "S"1. In contrast, by the hairy ball theorem, there is no (tangent) vector field on the 2-sphere "S"2 which is everywhere nonzero. K-theory studies the isomorphism classes of all vector bundles over some topological space. In addition to deepening topological and geometrical insight, it has purely algebraic consequences, such as the classification of finite-dimensional real division algebras: R, C, the quaternions H and the octonions O.
The cotangent bundle of a differentiable manifold consists, at every point of the manifold, of the dual of the tangent space, the cotangent space. Sections of that bundle are known as differential one-forms.
Modules.
"Modules" are to rings what vector spaces are to fields: the same axioms, applied to a ring "R" instead of a field "F", yield modules. The theory of modules, compared to that of vector spaces, is complicated by the presence of ring elements that do not have multiplicative inverses. For example, modules need not have bases, as the Z-module (that is, abelian group) Z/2Z shows; those modules that do (including all vector spaces) are known as free modules. Nevertheless, a vector space can be compactly defined as a module over a ring which is a field, with the elements being called vectors. Some authors use the term "vector space" to mean modules over a division ring. The algebro-geometric interpretation of commutative rings via their spectrum allows the development of concepts such as locally free modules, the algebraic counterpart to vector bundles.
Affine and projective spaces.
Roughly, "affine spaces" are vector spaces whose origins are not specified. More precisely, an affine space is a set with a free transitive vector space action. In particular, a vector space is an affine space over itself, by the map
If "W" is a vector space, then an affine subspace is a subset of "W" obtained by translating a linear subspace "V" by a fixed vector ; this space is denoted by (it is a coset of "V" in "W") and consists of all vectors of the form for An important example is the space of solutions of a system of inhomogeneous linear equations
generalizing the homogeneous case above, which can be found by setting in this equation. The space of solutions is the affine subspace where x is a particular solution of the equation, and "V" is the space of solutions of the homogeneous equation (the nullspace of "A").
The set of one-dimensional subspaces of a fixed finite-dimensional vector space "V" is known as "projective space"; it may be used to formalize the idea of parallel lines intersecting at infinity. Grassmannians and flag manifolds generalize this by parametrizing linear subspaces of fixed dimension "k" and flags of subspaces, respectively.