In
mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in modern mathematics ...
, a topological vector space (also called a linear topological space and commonly abbreviated TVS or t.v.s.) is one of the basic structures investigated in
functional analysis
Functional analysis is a branch of mathematical analysis, the core of which is formed by the study of vector spaces endowed with some kind of limit-related structure (e.g. inner product, norm, topology, etc.) and the linear functions defi ...
.
A topological vector space is a
vector space
In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...
that is also a
topological space
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 whose elements are called poin ...
with the property that the vector space operations (vector addition and scalar multiplication) are also
continuous functions. Such a topology is called a and every topological vector space has a
uniform topological structure, allowing a notion of
uniform convergence and
completeness. Some authors also require that the space is a
Hausdorff space
In topology and related branches of mathematics, a Hausdorff space ( , ), separated space or T2 space is a topological space where, for any two distinct points, there exist neighbourhoods of each which are disjoint from each other. Of the ma ...
(although this article does not). One of the most widely studied categories of TVSs are
locally convex topological vector space
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 ...
s. This article focuses on TVSs that are not necessarily locally convex.
Banach spaces,
Hilbert space
In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...
s and
Sobolev space
In mathematics, a Sobolev space is a vector space of functions equipped with a norm that is a combination of ''Lp''-norms of the function together with its derivatives up to a given order. The derivatives are understood in a suitable weak sense ...
s are other well-known examples of TVSs.
Many topological vector spaces are spaces of
functions, or
linear operators acting on topological vector spaces, and the topology is often defined so as to capture a particular notion of
convergence of sequences of functions.
In this article, the
scalar field of a topological vector space will be assumed to be either the
complex number
In mathematics, a complex number is an element of a number system that extends the real numbers with a specific element denoted , called the imaginary unit and satisfying the equation i^= -1; every complex number can be expressed in the fo ...
s
or the
real number
In mathematics, a real number is a number that can be used to measure a ''continuous'' one-dimensional quantity such as a distance, duration or temperature. Here, ''continuous'' means that values can have arbitrarily small variations. Every ...
s
unless clearly stated otherwise.
Motivation
Normed spaces
Every
normed vector space has a natural
topological structure: the norm induces a
metric
Metric or metrical may refer to:
* Metric system, an internationally adopted decimal system of measurement
* An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement
Mathematics
In mathe ...
and the metric induces a topology.
This is a topological vector space because:
#The vector addition map
defined by
is (jointly) continuous with respect to this topology. This follows directly from the
triangle inequality obeyed by the norm.
#The scalar multiplication map
defined by
where
is the underlying scalar field of
is (jointly) continuous. This follows from the triangle inequality and homogeneity of the norm.
Thus all
Banach spaces and
Hilbert space
In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...
s are examples of topological vector spaces.
Non-normed spaces
There are topological vector spaces whose topology is not induced by a norm, but are still of interest in analysis. Examples of such spaces are spaces of
holomorphic functions on an open domain, spaces of
infinitely differentiable functions, the
Schwartz spaces, and spaces of
test functions and the spaces of
distributions on them. These are all examples of
Montel spaces. An infinite-dimensional Montel space is never normable. The existence of a norm for a given topological vector space is characterized by
Kolmogorov's normability criterion.
A
topological field is a topological vector space over each of its
subfields.
Definition
A topological vector space (TVS)
is a
vector space
In mathematics and physics, a vector space (also called a linear space) is a set whose elements, often called '' vectors'', may be added together and multiplied ("scaled") by numbers called ''scalars''. Scalars are often real numbers, but can ...
over a
topological field (most often the
real or
complex numbers with their standard topologies) that is endowed with a
topology
In mathematics, topology (from the Greek words , and ) is concerned with the properties of a geometric object that are preserved under continuous deformations, such as stretching, twisting, crumpling, and bending; that is, without closing ...
such that vector addition
and scalar multiplication
are
continuous functions (where the domains of these functions are endowed with
product topologies). Such a topology is called a or a on
Every topological vector space is also a commutative
topological group under addition.
Hausdorff assumption
Many authors (for example,
Walter Rudin), but not this page, require the topology on
to be
T1; it then follows that the space is
Hausdorff, and even
Tychonoff. A topological vector space is said to be if it is Hausdorff; importantly, "separated" does not mean
separable. The topological and linear algebraic structures can be tied together even more closely with additional assumptions, the most common of which are listed
below
Below may refer to:
*Earth
* Ground (disambiguation)
*Soil
*Floor
* Bottom (disambiguation)
*Less than
*Temperatures below freezing
*Hell or underworld
People with the surname
*Ernst von Below (1863–1955), German World War I general
*Fred Below ...
.
Category and morphisms
The
category of topological vector spaces over a given topological field
is commonly denoted
or
The
objects
Object may refer to:
General meanings
* Object (philosophy), a thing, being, or concept
** Object (abstract), an object which does not exist at any particular time or place
** Physical object, an identifiable collection of matter
* Goal, an ai ...
are the topological vector spaces over
and the
morphisms are the
continuous -linear maps from one object to another.
A (abbreviated ), also called a , is a
continuous
Continuity or continuous may refer to:
Mathematics
* Continuity (mathematics), the opposing concept to discreteness; common examples include
** Continuous probability distribution or random variable in probability and statistics
** Continuous g ...
linear map
In mathematics, and more specifically in linear algebra, a linear map (also called a linear mapping, linear transformation, vector space homomorphism, or in some contexts linear function) is a mapping V \to W between two vector spaces that ...
between topological vector spaces (TVSs) such that the induced map
is an
open mapping
In mathematics, more specifically in topology, an open map is a function between two topological spaces that maps open sets to open sets.
That is, a function f : X \to Y is open if for any open set U in X, the image f(U) is open in Y.
Likewise, ...
when
which is the range or image of
is given the
subspace topology induced by
A (abbreviated ), also called a , is an
injective topological homomorphism. Equivalently, a TVS-embedding is a linear map that is also a
topological embedding.
A (abbreviated ), also called a or an , is a bijective
linear
Linearity is the property of a mathematical relationship ('' function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear ...
homeomorphism
In the mathematical field of topology, a homeomorphism, topological isomorphism, or bicontinuous function is a bijective and continuous function between topological spaces that has a continuous inverse function. Homeomorphisms are the isom ...
. Equivalently, it is a
surjective
In mathematics, a surjective function (also known as surjection, or onto function) is a function that every element can be mapped from element so that . In other words, every element of the function's codomain is the image of one element o ...
TVS embedding
Many properties of TVSs that are studied, such as
local convexity,
metrizability,
completeness, and
normability, are invariant under TVS isomorphisms.
A necessary condition for a vector topology
A collection
of subsets of a vector space is called if for every
there exists some
such that
All of the above conditions are consequently a necessity for a topology to form a vector topology.
Defining topologies using neighborhoods of the origin
Since every vector topology is translation invariant (which means that for all
the map
defined by
is a
homeomorphism
In the mathematical field of topology, a homeomorphism, topological isomorphism, or bicontinuous function is a bijective and continuous function between topological spaces that has a continuous inverse function. Homeomorphisms are the isom ...
), to define a vector topology it suffices to define a
neighborhood basis (or subbasis) for it at the origin.
In general, the set of all balanced and absorbing subsets of a vector space does not satisfy the conditions of this theorem and does not form a neighborhood basis at the origin for any vector topology.
Defining topologies using strings
Let
be a vector space and let
be a sequence of subsets of
Each set in the sequence
is called a of
and for every index
is called the
-th knot of
The set
is called the beginning of
The sequence
is/is a:
* if
for every index
*
Balanced (resp.
absorbing, closed,
[The topological properties of course also require that be a TVS.] convex, open,
symmetric,
barrelled,
absolutely convex/disked, etc.) if this is true of every
* if
is summative, absorbing, and balanced.
* or a in a TVS
if
is a string and each of its knots is a neighborhood of the origin in
If
is an
absorbing disk in a vector space
then the sequence defined by
forms a string beginning with
This is called the natural string of
Moreover, if a vector space
has countable dimension then every string contains an
absolutely convex string.
Summative sequences of sets have the particularly nice property that they define non-negative continuous real-valued
subadditive functions. These functions can then be used to prove many of the basic properties of topological vector spaces.
A proof of the above theorem is given in the article on
metrizable topological vector spaces.
If
and
are two collections of subsets of a vector space
and if
is a scalar, then by definition:
*
contains
:
if and only if
for every index
* Set of knots:
* Kernel:
* Scalar multiple:
* Sum:
* Intersection:
If
is a collection sequences of subsets of
then
is said to be directed (downwards) under inclusion or simply directed downward if
is not empty and for all
there exists some
such that
and
(said differently, if and only if
is a
prefilter with respect to the containment
defined above).
Notation: Let
be the set of all knots of all strings in
Defining vector topologies using collections of strings is particularly useful for defining classes of TVSs that are not necessarily locally convex.
If
is the set of all topological strings in a TVS
then
A Hausdorff TVS is
metrizable if and only if its topology can be induced by a single topological string.
Topological structure
A vector space is an
abelian group
In mathematics, an abelian group, also called a commutative group, is a group in which the result of applying the group operation to two group elements does not depend on the order in which they are written. That is, the group operation is comm ...
with respect to the operation of addition, and in a topological vector space the inverse operation is always continuous (since it is the same as multiplication by
). Hence, every topological vector space is an abelian
topological group. Every TVS is
completely regular but a TVS need not be
normal.
Let
be a topological vector space. Given a
subspace the quotient space
with the usual
quotient topology is a Hausdorff topological vector space if and only if
is closed.
[In particular, is Hausdorff if and only if the set is closed (that is, is a T1 space).] This permits the following construction: given a topological vector space
(that is probably not Hausdorff), form the quotient space
where
is the closure of
is then a Hausdorff topological vector space that can be studied instead of
Invariance of vector topologies
One of the most used properties of vector topologies is that every vector topology is :
:for all
the map
defined by
is a
homeomorphism
In the mathematical field of topology, a homeomorphism, topological isomorphism, or bicontinuous function is a bijective and continuous function between topological spaces that has a continuous inverse function. Homeomorphisms are the isom ...
, but if
then it is not linear and so not a TVS-isomorphism.
Scalar multiplication by a non-zero scalar is a TVS-isomorphism. This means that if
then the linear map
defined by
is a homeomorphism. Using
produces the negation map
defined by
which is consequently a linear homeomorphism and thus a TVS-isomorphism.
If
and any subset
then
and moreover, if
then
is a
neighborhood (resp. open neighborhood, closed neighborhood) of
in
if and only if the same is true of
at the origin.
Local notions
A subset
of a vector space
is said to be
*
absorbing (in
): if for every
there exists a real
such that
for any scalar
satisfying
*
balanced or circled: if
for every scalar
*
convex: if
for every real
* a
disk or
absolutely convex: if
is convex and balanced.
*
symmetric: if
or equivalently, if
Every neighborhood of the origin is an
absorbing set and contains an open
balanced neighborhood of
so every topological vector space has a local base of absorbing and
balanced sets. The origin even has a neighborhood basis consisting of closed balanced neighborhoods of
if the space is
locally convex then it also has a neighborhood basis consisting of closed convex balanced neighborhoods of the origin.
Bounded subsets
A subset
of a topological vector space
is
bounded if for every neighborhood
of the origin, then
when
is sufficiently large.
The definition of boundedness can be weakened a bit;
is bounded if and only if every countable subset of it is bounded. A set is bounded if and only if each of its subsequences is a bounded set. Also,
is bounded if and only if for every balanced neighborhood
of the origin, there exists
such that
Moreover, when
is locally convex, the boundedness can be characterized by
seminorm In mathematics, particularly in functional analysis, a seminorm is a vector space norm that need not be positive definite. Seminorms are intimately connected with convex sets: every seminorm is the Minkowski functional of some absorbing disk a ...
s: the subset
is bounded if and only if every continuous seminorm
is bounded on
Every
totally bounded set is bounded. If
is a vector subspace of a TVS
then a subset of
is bounded in
if and only if it is bounded in
Metrizability
A TVS is
pseudometrizable if and only if it has a countable neighborhood basis at the origin, or equivalent, if and only if its topology is generated by an
''F''-seminorm. A TVS is metrizable if and only if it is Hausdorff and pseudometrizable.
More strongly: a topological vector space is said to be
normable
In mathematics, a norm is a function from a real or complex vector space to the non-negative real numbers that behaves in certain ways like the distance from the origin: it commutes with scaling, obeys a form of the triangle inequality, and is z ...
if its topology can be induced by a norm. A topological vector space is normable if and only if it is Hausdorff and has a convex bounded neighborhood of the origin.
Let
be a non-
discrete locally compact topological field, for example the real or complex numbers. A
Hausdorff topological vector space over
is locally compact if and only if it is
finite-dimensional
In mathematics, the dimension of a vector space ''V'' is the cardinality (i.e., the number of vectors) of a basis of ''V'' over its base field. p. 44, §2.36 It is sometimes called Hamel dimension (after Georg Hamel) or algebraic dimension to d ...
, that is, isomorphic to
for some natural number
Completeness and uniform structure
The
canonical uniformity on a TVS
is the unique translation-invariant
uniformity that induces the topology
on
Every TVS is assumed to be endowed with this canonical uniformity, which makes all TVSs into
uniform spaces. This allows one to about related notions such as
completeness,
uniform convergence, Cauchy nets, and
uniform continuity. etc., which are always assumed to be with respect to this uniformity (unless indicated other). This implies that every Hausdorff topological vector space is
Tychonoff. A subspace of a TVS is
compact
Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to:
* Interstate compact
* Blood compact, an ancient ritual of the Philippines
* Compact government, a type of colonial rule utilized in Britis ...
if and only if it is complete and
totally bounded (for Hausdorff TVSs, a set being totally bounded is equivalent to it being
precompact). But if the TVS is not Hausdorff then there exist compact subsets that are not closed. However, the closure of a compact subset of a non-Hausdorff TVS is again compact (so compact subsets are
relatively compact
In mathematics, a relatively compact subspace (or relatively compact subset, or precompact subset) of a topological space is a subset whose closure is compact.
Properties
Every subset of a compact topological space is relatively compact (sinc ...
).
With respect to this uniformity, a
net (or
sequence
In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called ...
)
is Cauchy if and only if for every neighborhood
of
there exists some index
such that
whenever
and
Every Cauchy sequence is bounded, although Cauchy nets and Cauchy filters may not be bounded. A topological vector space where every Cauchy sequence converges is called
sequentially complete
In mathematics, specifically in topology and functional analysis, a subspace of a uniform space is said to be sequentially complete or semi-complete if every Cauchy sequence in converges to an element in .
is called sequentially complete if i ...
; in general, it may not be complete (in the sense that all Cauchy filters converge).
The vector space operation of addition is uniformly continuous and an
open map. Scalar multiplication is
Cauchy continuous In mathematics, a Cauchy-continuous, or Cauchy-regular, function is a special kind of continuous function between metric spaces (or more general spaces). Cauchy-continuous functions have the useful property that they can always be (uniquely) exten ...
but in general, it is almost never uniformly continuous. Because of this, every topological vector space can be completed and is thus a
dense linear subspace
In mathematics, and more specifically in linear algebra, a linear subspace, also known as a vector subspaceThe term ''linear subspace'' is sometimes used for referring to flats and affine subspaces. In the case of vector spaces over the reals, l ...
of a
complete 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 x towards ...
.
* Every TVS has a
completion and every Hausdorff TVS has a Hausdorff completion. Every TVS (even those that are Hausdorff and/or complete) has infinitely many non-isomorphic non-Hausdorff completions.
* A compact subset of a TVS (not necessarily Hausdorff) is complete. A complete subset of a Hausdorff TVS is closed.
* If
is a complete subset of a TVS then any subset of
that is closed in
is complete.
* A Cauchy sequence in a Hausdorff TVS
is not necessarily
relatively compact
In mathematics, a relatively compact subspace (or relatively compact subset, or precompact subset) of a topological space is a subset whose closure is compact.
Properties
Every subset of a compact topological space is relatively compact (sinc ...
(that is, its closure in
is not necessarily compact).
* If a Cauchy filter in a TVS has an
accumulation point then it converges to
* If a series
converges
[A series is said to converge in a TVS if the sequence of partial sums converges.] in a TVS
then
in
Examples
Finest and coarsest vector topology
Let
be a real or complex vector space.
Trivial topology
The
trivial topology or indiscrete topology
is always a TVS topology on any vector space
and it is the coarsest TVS topology possible. An important consequence of this is that the intersection of any collection of TVS topologies on
always contains a TVS topology. Any vector space (including those that are infinite dimensional) endowed with the trivial topology is a compact (and thus
locally compact)
complete
Complete may refer to:
Logic
* Completeness (logic)
* Completeness of a theory, the property of a theory that every formula in the theory's language or its negation is provable
Mathematics
* The completeness of the real numbers, which implies t ...
pseudometrizable seminormable locally convex topological vector space. It is
Hausdorff if and only if
Finest vector topology
There exists a TVS topology
on
called the on
that is finer than every other TVS-topology on
(that is, any TVS-topology on
is necessarily a subset of
).
Every linear map from
into another TVS is necessarily continuous. If
has an uncountable
Hamel basis then
is
locally convex and
metrizable.
Cartesian products
A
Cartesian product
In mathematics, specifically set theory, the Cartesian product of two sets ''A'' and ''B'', denoted ''A''×''B'', is the set of all ordered pairs where ''a'' is in ''A'' and ''b'' is in ''B''. In terms of set-builder notation, that is
: A\t ...
of a family of topological vector spaces, when endowed with the
product topology
In topology and related areas of mathematics, a product space is the Cartesian product of a family of topological spaces equipped with a natural topology called the product topology. This topology differs from another, perhaps more natural-seem ...
, is a topological vector space. Consider for instance the set
of all functions
where
carries its usual
Euclidean topology
In mathematics, and especially general topology, the Euclidean topology is the natural topology induced on n-dimensional Euclidean space
Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, ...
. This set
is a real vector space (where addition and scalar multiplication are defined pointwise, as usual) that can be identified with (and indeed, is often defined to be) the
Cartesian product
In mathematics, specifically set theory, the Cartesian product of two sets ''A'' and ''B'', denoted ''A''×''B'', is the set of all ordered pairs where ''a'' is in ''A'' and ''b'' is in ''B''. In terms of set-builder notation, that is
: A\t ...
which carries the natural
product topology
In topology and related areas of mathematics, a product space is the Cartesian product of a family of topological spaces equipped with a natural topology called the product topology. This topology differs from another, perhaps more natural-seem ...
. With this product topology,
becomes a topological vector space whose topology is called The reason for this name is the following: if
is a
sequence
In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order matters. Like a set, it contains members (also called ''elements'', or ''terms''). The number of elements (possibly infinite) is called ...
(or more generally, a
net) of elements in
and if
then
converges to
in
if and only if for every real number
converges to
in
This TVS is
complete
Complete may refer to:
Logic
* Completeness (logic)
* Completeness of a theory, the property of a theory that every formula in the theory's language or its negation is provable
Mathematics
* The completeness of the real numbers, which implies t ...
,
Hausdorff, and
locally convex but not
metrizable and consequently not
normable
In mathematics, a norm is a function from a real or complex vector space to the non-negative real numbers that behaves in certain ways like the distance from the origin: it commutes with scaling, obeys a form of the triangle inequality, and is z ...
; indeed, every neighborhood of the origin in the product topology contains lines (that is, 1-dimensional vector subspaces, which are subsets of the form
with
).
Finite-dimensional spaces
By
F. Riesz's theorem, a Hausdorff topological vector space is finite-dimensional if and only if it is
locally compact, which happens if and only if it has a compact
neighborhood of the origin.
Let
denote
or
and endow
with its usual Hausdorff normed
Euclidean topology
In mathematics, and especially general topology, the Euclidean topology is the natural topology induced on n-dimensional Euclidean space
Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, ...
. Let
be a vector space over
of finite dimension
and so that
is vector space isomorphic to
(explicitly, this means that there exists a
linear isomorphism between the vector spaces
and
). This finite-dimensional vector space
always has a unique vector topology, which makes it TVS-isomorphic to
where
is endowed with the usual Euclidean topology (which is the same as the
product topology
In topology and related areas of mathematics, a product space is the Cartesian product of a family of topological spaces equipped with a natural topology called the product topology. This topology differs from another, perhaps more natural-seem ...
). This Hausdorff vector topology is also the (unique)
finest vector topology on
has a unique vector topology if and only if
If
then although
does not have a unique vector topology, it does have a unique vector topology.
* If
then
has exactly one vector topology: the
trivial topology, which in this case (and in this case) is Hausdorff. The trivial topology on a vector space is Hausdorff if and only if the vector space has dimension
* If
then
has two vector topologies: the usual
Euclidean topology
In mathematics, and especially general topology, the Euclidean topology is the natural topology induced on n-dimensional Euclidean space
Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, ...
and the (non-Hausdorff) trivial topology.
** Since the field
is itself a
-dimensional topological vector space over
and since it plays an important role in the definition of topological vector spaces, this dichotomy plays an important role in the definition of an
absorbing set and has consequences that reverberate throughout
functional analysis
Functional analysis is a branch of mathematical analysis, the core of which is formed by the study of vector spaces endowed with some kind of limit-related structure (e.g. inner product, norm, topology, etc.) and the linear functions defi ...
.
* If
then
has distinct vector topologies:
** Some of these topologies are now described: Every linear functional
on
which is vector space isomorphic to
induces a
seminorm In mathematics, particularly in functional analysis, a seminorm is a vector space norm that need not be positive definite. Seminorms are intimately connected with convex sets: every seminorm is the Minkowski functional of some absorbing disk a ...
defined by
where
Every seminorm induces a (
pseudometrizable locally convex) vector topology on
and seminorms with distinct kernels induce distinct topologies so that in particular, seminorms on
that are induced by linear functionals with distinct kernel will induces distinct vector topologies on
** However, while there are infinitely many vector topologies on
when
there are, only
vector topologies on
For instance, if
then the vector topologies on
consist of the trivial topology, the Hausdorff Euclidean topology, and then the infinitely many remaining non-trivial non-Euclidean vector topologies on
are all TVS-isomorphic to one another.
Non-vector topologies
Discrete and cofinite topologies
If
is a non-trivial vector space (that is, of non-zero dimension) then the
discrete topology on
(which is always
metrizable) is a TVS topology because despite making addition and negation continuous (which makes it into a
topological group under addition), it fails to make scalar multiplication continuous. The
cofinite topology on
(where a subset is open if and only if its complement is finite) is also a TVS topology on
Linear maps
A linear operator between two topological vector spaces which is continuous at one point is continuous on the whole domain. Moreover, a linear operator
is continuous if
is bounded (as defined below) for some neighborhood
of the origin.
A
hyperplane on a topological vector space
is either dense or closed. A
linear functional on a topological vector space
has either dense or closed kernel. Moreover,
is continuous if and only if its
kernel is
closed
Closed may refer to:
Mathematics
* Closure (mathematics), a set, along with operations, for which applying those operations on members always results in a member of the set
* Closed set, a set which contains all its limit points
* Closed interval, ...
.
Types
Depending on the application additional constraints are usually enforced on the topological structure of the space. In fact, several principal results in functional analysis fail to hold in general for topological vector spaces: the
closed graph theorem, the
open mapping theorem, and the fact that the dual space of the space separates points in the space.
Below are some common topological vector spaces, roughly in order of increasing "niceness."
*
F-spaces are
complete
Complete may refer to:
Logic
* Completeness (logic)
* Completeness of a theory, the property of a theory that every formula in the theory's language or its negation is provable
Mathematics
* The completeness of the real numbers, which implies t ...
topological vector spaces with a translation-invariant metric. These include
spaces for all
*
Locally convex topological vector space
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 ...
s: here each point has a
local base consisting of
convex set
In geometry, a subset of a Euclidean space, or more generally an affine space over the reals, is convex if, given any two points in the subset, the subset contains the whole line segment that joins them. Equivalently, a convex set or a convex ...
s. By a technique known as
Minkowski functional
In mathematics, in the field of functional analysis, a Minkowski functional (after Hermann Minkowski) or gauge function is a function that recovers a notion of distance on a linear space.
If K is a subset of a real or complex vector space X, t ...
s it can be shown that a space is locally convex if and only if its topology can be defined by a family of seminorms. Local convexity is the minimum requirement for "geometrical" arguments like the
Hahn–Banach theorem. The
spaces are locally convex (in fact, Banach spaces) for all
but not for
*
Barrelled spaces: locally convex spaces where the
Banach–Steinhaus theorem holds.
*
Bornological space: a locally convex space where the
continuous linear operators to any locally convex space are exactly the
bounded linear operators.
*
Stereotype space In the area of mathematics known as functional analysis, a reflexive space is a locally convex topological vector space (TVS) for which the canonical evaluation map from X into its bidual (which is the strong dual of the strong dual of X) is an iso ...
: a locally convex space satisfying a variant of
reflexivity condition, where the dual space is endowed with the topology of uniform convergence on
totally bounded sets.
*
Montel space: a barrelled space where every
closed
Closed may refer to:
Mathematics
* Closure (mathematics), a set, along with operations, for which applying those operations on members always results in a member of the set
* Closed set, a set which contains all its limit points
* Closed interval, ...
and
bounded set
:''"Bounded" and "boundary" are distinct concepts; for the latter see boundary (topology). A circle in isolation is a boundaryless bounded set, while the half plane is unbounded yet has a boundary.
In mathematical analysis and related areas of m ...
is
compact
Compact as used in politics may refer broadly to a pact or treaty; in more specific cases it may refer to:
* Interstate compact
* Blood compact, an ancient ritual of the Philippines
* Compact government, a type of colonial rule utilized in Britis ...
*
Fréchet spaces: these are complete locally convex spaces where the topology comes from a translation-invariant metric, or equivalently: from a countable family of seminorms. Many interesting spaces of functions fall into this class --
is a Fréchet space under the seminorms
A locally convex F-space is a Fréchet space.
*
LF-space In mathematics, an ''LF''-space, also written (''LF'')-space, is a topological vector space (TVS) ''X'' that is a locally convex inductive limit of a countable inductive system (X_n, i_) of Fréchet spaces.
This means that ''X'' is a direct li ...
s are
limits of
Fréchet spaces.
ILH spaces are
inverse limits of Hilbert spaces.
*
Nuclear spaces: these are locally convex spaces with the property that every bounded map from the nuclear space to an arbitrary Banach space is a
nuclear operator.
*
Normed spaces and
seminormed spaces: locally convex spaces where the topology can be described by a single
norm or
seminorm In mathematics, particularly in functional analysis, a seminorm is a vector space norm that need not be positive definite. Seminorms are intimately connected with convex sets: every seminorm is the Minkowski functional of some absorbing disk a ...
. In normed spaces a linear operator is continuous if and only if it is bounded.
*
Banach spaces: Complete
normed vector spaces. Most of functional analysis is formulated for Banach spaces. This class includes the
spaces with
the space
of
functions of bounded variation, and
certain spaces of measures.
*
Reflexive Banach space In the area of mathematics known as functional analysis, a reflexive space is a locally convex topological vector space (TVS) for which the canonical evaluation map from X into its bidual (which is the strong dual of the strong dual of X) is an iso ...
s: Banach spaces naturally isomorphic to their double dual (see below), which ensures that some geometrical arguments can be carried out. An important example which is reflexive is
, whose dual is
but is strictly contained in the dual of
*
Hilbert space
In mathematics, Hilbert spaces (named after David Hilbert) allow generalizing the methods of linear algebra and calculus from (finite-dimensional) Euclidean vector spaces to spaces that may be infinite-dimensional. Hilbert spaces arise natural ...
s: these have an
inner product
In mathematics, an inner product space (or, rarely, a Hausdorff pre-Hilbert space) is a real vector space or a complex vector space with an operation called an inner product. The inner product of two vectors in the space is a scalar, often ...
; even though these spaces may be infinite-dimensional, most geometrical reasoning familiar from finite dimensions can be carried out in them. These include
spaces, the
Sobolev spaces and
Hardy spaces.
*
Euclidean space
Euclidean space is the fundamental space of geometry, intended to represent physical space. Originally, that is, in Euclid's ''Elements'', it was the three-dimensional space of Euclidean geometry, but in modern mathematics there are Euclidea ...
s:
or
with the topology induced by the standard inner product. As pointed out in the preceding section, for a given finite
there is only one
-dimensional topological vector space, up to isomorphism. It follows from this that any finite-dimensional subspace of a TVS is closed. A characterization of finite dimensionality is that a Hausdorff TVS is locally compact if and only if it is finite-dimensional (therefore isomorphic to some Euclidean space).
Dual space
Every topological vector space has a
continuous dual space—the set
of all continuous linear functionals, that is,
continuous linear maps from the space into the base field
A topology on the dual can be defined to be the coarsest topology such that the dual pairing each point evaluation
is continuous. This turns the dual into a locally convex topological vector space. This topology is called the
weak-* topology
In mathematics, weak topology is an alternative term for certain initial topologies, often on topological vector spaces or spaces of linear operators, for instance on a Hilbert space. The term is most commonly used for the initial topology of a ...
. This may not be the only natural topology on the dual space; for instance, the dual of a normed space has a natural norm defined on it. However, it is very important in applications because of its compactness properties (see
Banach–Alaoglu theorem
In functional analysis and related branches of mathematics, the Banach–Alaoglu theorem (also known as Alaoglu's theorem) states that the closed unit ball of the dual space of a normed vector space is compact in the weak* topology.
A common p ...
). Caution: Whenever
is a non-normable locally convex space, then the pairing map
is never continuous, no matter which vector space topology one chooses on
A topological vector space has a non-trivial continuous dual space if and only if it has a proper convex neighborhood of the origin.
Properties
For any
of a TVS
the
''convex'' (resp. ''
balanced,
disked, closed convex, closed balanced, closed disked) ''hull'' of
is the smallest subset of
that has this property and contains
The closure (respectively, interior,
convex hull, balanced hull, disked hull) of a set
is sometimes denoted by
(respectively,
).
The
convex hull of a subset
is equal to the set of all of elements in
which are finite
linear combinations of the form
where
is an integer,
and