TheInfoList

In
mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and their changes (cal ...
, and more specifically in
linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces and through matrix (mat ...
, a linear subspace, also known as a vector subspaceThe term ''linear subspace'' is sometimes used for referring to
flats Flat or flats may refer to: Architecture * Flat (housing), an apartment in the United Kingdom, Ireland, Australia and other Commonwealth countries Arts and entertainment * Flat (music), a symbol () which denotes a lower pitch * Flat (soldier), a ...
and
affine subspace In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). ...

s. In the case of vector spaces over the reals, linear subspaces, flats, and affine subspaces are also called ''linear manifolds'' for emphasizing that there are also
manifold In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities a ...

s.
is a
vector space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities a ...
that is a
subset In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities a ...

of some larger vector space. A linear subspace is usually simply called a ''subspace'' when the context serves to distinguish it from other types of subspaces.

# Definition

If ''V'' is a vector space over a
field Field may refer to: Expanses of open ground * Field (agriculture), an area of land used for agricultural purposes * Airfield, an aerodrome that lacks the infrastructure of an airport * Battlefield * Lawn, an area of mowed grass * Meadow, a grassl ...
''K'' and if ''W'' is a subset of ''V'', then ''W'' is a linear subspace of ''V'' if under the operations of ''V'', ''W'' is a vector space over ''K''. Equivalently, a
nonempty #REDIRECT Empty set #REDIRECT Empty set#REDIRECT Empty set In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry ...

subset ''W'' is a subspace of ''V'' if, whenever are elements of ''W'' and are elements of ''K'', it follows that is in ''W''. As a corollary, all vector spaces are equipped with at least two (possibly different) linear subspaces: the
zero vector space Morphisms to and from the zero object In algebra Algebra (from ar, الجبر, lit=reunion of broken parts, bonesetting, translit=al-jabr) is one of the areas of mathematics, broad areas of mathematics, together with number theory, geomet ...
consisting of the
zero vector In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...
alone and the entire vector space itself. These are called the trivial subspaces of the vector space.

# Examples

## Example I

Let the field ''K'' be the set R of
real number In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no g ...
s, and let the vector space ''V'' be the
real coordinate space In mathematics, a real coordinate space of dimension , written ( ) or is a coordinate space over the real numbers. This means that it is the set of the tuple, -tuples of real numbers (sequences of real numbers). With component-wise addition a ...
R3. Take ''W'' to be the set of all vectors in ''V'' whose last component is 0. Then ''W'' is a subspace of ''V''. ''Proof:'' #Given u and v in ''W'', then they can be expressed as and . Then . Thus, u + v is an element of ''W'', too. #Given u in ''W'' and a scalar ''c'' in R, if again, then . Thus, ''c''u is an element of ''W'' too.

## Example II

Let the field be R again, but now let the vector space ''V'' be the
Cartesian plane A Cartesian coordinate system (, ) in a plane Plane or planes may refer to: * Airplane An airplane or aeroplane (informally plane) is a fixed-wing aircraft A fixed-wing aircraft is a heavier-than-air flying machine Early fly ...

R2. Take ''W'' to be the set of points (''x'', ''y'') of R2 such that ''x'' = ''y''. Then ''W'' is a subspace of R2. ''Proof:'' #Let and be elements of ''W'', that is, points in the plane such that ''p''1 = ''p''2 and ''q''1 = ''q''2. Then ; since ''p''1 = ''p''2 and ''q''1 = ''q''2, then ''p''1 + ''q''1 = ''p''2 + ''q''2, so p + q is an element of ''W''. #Let p = (''p''1, ''p''2) be an element of ''W'', that is, a point in the plane such that ''p''1 = ''p''2, and let ''c'' be a scalar in R. Then ; since ''p''1 = ''p''2, then ''cp''1 = ''cp''2, so ''c''p is an element of ''W''. In general, any subset of the real coordinate space R''n'' that is defined by a system of homogeneous
linear equation In mathematics, a linear equation is an equation that may be put in the form :a_1x_1+\cdots +a_nx_n+b=0, where x_1, \ldots, x_n are the variable (mathematics), variables (or unknown (mathematics), unknowns), and b, a_1, \ldots, a_n are the coeffi ...

s will yield a subspace. (The equation in example I was ''z'' = 0, and the equation in example II was ''x'' = ''y''.) Geometrically, these subspaces are points, lines, planes and spaces that pass through the point 0.

## Example III

Again take the field to be R, but now let the vector space ''V'' be the set RR of all
function Function or functionality may refer to: Computing * Function key A function key is a key on a computer A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations automatically. Modern comp ...
s from R to R. Let C(R) be the subset consisting of
continuous function In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
s. Then C(R) is a subspace of RR. ''Proof:'' #We know from calculus that . #We know from calculus that the sum of continuous functions is continuous. #Again, we know from calculus that the product of a continuous function and a number is continuous.

## Example IV

Keep the same field and vector space as before, but now consider the set Diff(R) of all
differentiable function In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...

s. The same sort of argument as before shows that this is a subspace too. Examples that extend these themes are common in
functional analysis Functional analysis is a branch of mathematical analysis Analysis is the branch of mathematics dealing with Limit (mathematics), limits and related theories, such as Derivative, differentiation, Integral, integration, Measure (mathematics), ...
.

# Properties of subspaces

From the definition of vector spaces, it follows that subspaces are nonempty, and are closed under sums and under scalar multiples. Equivalently, subspaces can be characterized by the property of being closed under linear combinations. That is, a nonempty set ''W'' is a subspace
if and only if In logic Logic is an interdisciplinary field which studies truth and reasoning. Informal logic seeks to characterize Validity (logic), valid arguments informally, for instance by listing varieties of fallacies. Formal logic represents st ...
every linear combination of
finite Finite is the opposite of infinite Infinite may refer to: Mathematics *Infinite set, a set that is not a finite set *Infinity, an abstract concept describing something without any limit Music *Infinite (band), a South Korean boy band *''Infin ...
ly many elements of ''W'' also belongs to ''W''. The equivalent definition states that it is also equivalent to consider linear combinations of two elements at a time. In a
topological vector space In 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. A topological vector space is a vector space (an Abstra ...
''X'', a subspace ''W'' need not be topologically closed, but a
finite-dimensional In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and th ...
subspace is always closed. The same is true for subspaces of finite
codimension In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and th ...
(i.e., subspaces determined by a finite number of continuous
linear functional In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and t ...
s).

# Descriptions

Descriptions of subspaces include the solution set to a homogeneous
system of linear equations In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
, the subset of Euclidean space described by a system of homogeneous linear
parametric equations In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
, the span of a collection of vectors, and the
null space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
,
column space In linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces a ...
, and
row space In linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces a ...
of a
matrix Matrix or MATRIX may refer to: Science and mathematics * Matrix (mathematics), a rectangular array of numbers, symbols, or expressions * Matrix (logic), part of a formula in prenex normal form * Matrix (biology), the material in between a eukaryot ...
. Geometrically (especially over the field of real numbers and its subfields), a subspace is a
flat Flat or flats may refer to: Architecture * Flat (housing), an apartment in the United Kingdom, Ireland, Australia and other Commonwealth countries Arts and entertainment * Flat (music), a symbol () which denotes a lower pitch * Flat (soldier), a ...
in an ''n''-space that passes through the origin. A natural description of a 1-subspace is the
scalar multiplication In mathematics, scalar multiplication is one of the basic operations defining a vector space in linear algebra (or more generally, a module (mathematics), module in abstract algebra). In common geometrical contexts, scalar multiplication of a re ...

of one non-
zero 0 (zero) is a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is anything that has been (or could be) formally defined, and ...
vector v to all possible scalar values. 1-subspaces specified by two vectors are equal if and only if one vector can be obtained from another with scalar multiplication: :$\exist c\in K: \mathbf\text{'} = c\mathbf\text\mathbf = \frac\mathbf\text{'}\text$ This idea is generalized for higher dimensions with
linear span In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and th ...
, but criteria for
equality Equality may refer to: Society * Political equality, in which all members of a society are of equal standing ** Consociationalism, in which an ethnically, religiously, or linguistically divided state functions by cooperation of each group's elite ...
of ''k''-spaces specified by sets of ''k'' vectors are not so simple. A
dual Dual or Duals may refer to: Paired/two things * Dual (mathematics), a notion of paired concepts that mirror one another ** Dual (category theory), a formalization of mathematical duality ** . . . see more cases in :Duality theories * Dual ...
description is provided with
linear functionals In mathematics, a linear form (also known as a linear functional, a one-form, or a covector) is a linear map from a vector space to its field (mathematics), field of scalar (mathematics), scalars (often, the real numbers or the complex numbers). ...
(usually implemented as linear equations). One non-
zero 0 (zero) is a number A number is a mathematical object A mathematical object is an abstract concept arising in mathematics. In the usual language of mathematics, an ''object'' is anything that has been (or could be) formally defined, and ...
linear functional F specifies its
kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learnin ...
subspace F = 0 of codimension 1. Subspaces of codimension 1 specified by two linear functionals are equal, if and only if one functional can be obtained from another with scalar multiplication (in the
dual space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers ( and ), formulas and related structures (), shapes and spaces in which they are contained (), and quantities and their changes ( and ). There is no gen ...
): :$\exist c\in K: \mathbf\text{'} = c\mathbf\text\mathbf = \frac\mathbf\text{'}\text$ It is generalized for higher codimensions with a
system of equations In mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It h ...
. The following two subsections will present this latter description in details, and the remaining four subsections further describe the idea of linear span.

## Systems of linear equations

The solution set to any homogeneous
system of linear equations In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
with ''n'' variables is a subspace in the
coordinate space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities a ...
''K''''n'': :$\left\.$ For example, the set of all vectors (''x'', ''y'', ''z'') (over real or
rational number In mathematics, a rational number is a number that can be expressed as the quotient or fraction (mathematics), fraction of two integers, a numerator and a non-zero denominator . For example, is a rational number, as is every integer (e.g. ) ...
s) satisfying the equations :$x + 3y + 2z = 0 \;\;\;\;\text\;\;\;\; 2x - 4y + 5z = 0$ is a one-dimensional subspace. More generally, that is to say that given a set of ''n'' independent functions, the dimension of the subspace in ''K''''k'' will be the dimension of the
null set In mathematical analysis Analysis is the branch of mathematics dealing with Limit (mathematics), limits and related theories, such as Derivative, differentiation, Integral, integration, Measure (mathematics), measure, sequences, Series (mathema ...
of ''A'', the composite matrix of the ''n'' functions.

## Null space of a matrix

In a finite-dimensional space, a homogeneous system of linear equations can be written as a single matrix equation: :$A\mathbf = \mathbf.$ The set of solutions to this equation is known as the
null space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
of the matrix. For example, the subspace described above is the null space of the matrix :$A = \begin 1 & 3 & 2 \\ 2 & -4 & 5 \end .$ Every subspace of ''K''''n'' can be described as the null space of some matrix (see below for more).

## Linear parametric equations

The subset of ''K''''n'' described by a system of homogeneous linear
parametric equations In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
is a subspace: :$\left\.$ For example, the set of all vectors (''x'', ''y'', ''z'') parameterized by the equations :$x = 2t_1 + 3t_2,\;\;\;\;y = 5t_1 - 4t_2,\;\;\;\;\text\;\;\;\;z = -t_1 + 2t_2$ is a two-dimensional subspace of ''K''3, if ''K'' is a
number field In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and th ...
(such as real or rational numbers).Generally, ''K'' can be any field of such
characteristic Characteristic (from the Greek word for a property, attribute or trait Trait may refer to: * Phenotypic trait in biology, which involve genes and characteristics of organisms * Trait (computer programming), a model for structuring object-oriented ...
that the given integer matrix has the appropriate
rank Rank is the relative position, value, worth, complexity, power, importance, authority, level, etc. of a person or object within a ranking A ranking is a relationship between a set of items such that, for any two items, the first is either "rank ...
in it. All fields include
integer An integer (from the Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', meaning "to share" or "to ...
s, but some integers may equal to zero in some fields.

## Span of vectors

In linear algebra, the system of parametric equations can be written as a single vector equation: :$\begin x \\ y \\ z \end \;=\; t_1 \!\begin 2 \\ 5 \\ -1 \end + t_2 \!\begin 3 \\ -4 \\ 2 \end.$ The expression on the right is called a linear combination of the vectors (2, 5, −1) and (3, −4, 2). These two vectors are said to span the resulting subspace. In general, a linear combination of vectors v1, v2, ... , v''k'' is any vector of the form :$t_1 \mathbf_1 + \cdots + t_k \mathbf_k.$ The set of all possible linear combinations is called the span: :$\text \ = \left\ .$ If the vectors v1, ... , v''k'' have ''n'' components, then their span is a subspace of ''K''''n''. Geometrically, the span is the flat through the origin in ''n''-dimensional space determined by the points v1, ... , v''k''. ; Example : The ''xz''-plane in R3 can be parameterized by the equations ::$x = t_1, \;\;\; y = 0, \;\;\; z = t_2.$ :As a subspace, the ''xz''-plane is spanned by the vectors (1, 0, 0) and (0, 0, 1). Every vector in the ''xz''-plane can be written as a linear combination of these two: ::$\left(t_1, 0, t_2\right) = t_1\left(1,0,0\right) + t_2\left(0,0,1\right)\text$ :Geometrically, this corresponds to the fact that every point on the ''xz''-plane can be reached from the origin by first moving some distance in the direction of (1, 0, 0) and then moving some distance in the direction of (0, 0, 1).

## Column space and row space

A system of linear parametric equations in a finite-dimensional space can also be written as a single matrix equation: : In this case, the subspace consists of all possible values of the vector x. In linear algebra, this subspace is known as the column space (or
image An image (from la, imago) is an artifact that depicts visual perception Visual perception is the ability to interpret the surrounding environment (biophysical), environment through photopic vision (daytime vision), color vision, sco ...
) of the matrix ''A''. It is precisely the subspace of ''K''''n'' spanned by the column vectors of ''A''. The row space of a matrix is the subspace spanned by its row vectors. The row space is interesting because it is the
orthogonal complement In the mathematical Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities an ...
of the null space (see below).

## Independence, basis, and dimension

In general, a subspace of ''K''''n'' determined by ''k'' parameters (or spanned by ''k'' vectors) has dimension ''k''. However, there are exceptions to this rule. For example, the subspace of ''K''3 spanned by the three vectors (1, 0, 0), (0, 0, 1), and (2, 0, 3) is just the ''xz''-plane, with each point on the plane described by infinitely many different values of . In general, vectors v1, ... , v''k'' are called linearly independent if :$t_1 \mathbf_1 + \cdots + t_k \mathbf_k \;\ne\; u_1 \mathbf_1 + \cdots + u_k \mathbf_k$ for (''t''1, ''t''2, ... , ''tk'') ≠ (''u''1, ''u''2, ... , ''uk'').This definition is often stated differently: vectors v1, ..., v''k'' are linearly independent if for . The two definitions are equivalent. If are linearly independent, then the coordinates for a vector in the span are uniquely determined. A basis for a subspace ''S'' is a set of linearly independent vectors whose span is ''S''. The number of elements in a basis is always equal to the geometric dimension of the subspace. Any spanning set for a subspace can be changed into a basis by removing redundant vectors (see § Algorithms below for more). ; Example : Let ''S'' be the subspace of R4 defined by the equations ::$x_1 = 2 x_2\;\;\;\;\text\;\;\;\;x_3 = 5x_4.$ :Then the vectors (2, 1, 0, 0) and (0, 0, 5, 1) are a basis for ''S''. In particular, every vector that satisfies the above equations can be written uniquely as a linear combination of the two basis vectors: ::$\left(2t_1, t_1, 5t_2, t_2\right) = t_1\left(2, 1, 0, 0\right) + t_2\left(0, 0, 5, 1\right).$ :The subspace ''S'' is two-dimensional. Geometrically, it is the plane in R4 passing through the points (0, 0, 0, 0), (2, 1, 0, 0), and (0, 0, 5, 1).

# Operations and relations on subspaces

## Inclusion

The set-theoretical inclusion binary relation specifies a
partial order In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
on the set of all subspaces (of any dimension). A subspace cannot lie in any subspace of lesser dimension. If dim ''U'' = ''k'', a finite number, and ''U'' ⊂ ''W'', then dim ''W'' = ''k'' if and only if ''U'' = ''W''.

## Intersection

Given subspaces ''U'' and ''W'' of a vector space ''V'', then their
intersection In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities ...
''U'' ∩ ''W'' := is also a subspace of ''V''. ''Proof:'' # Let v and w be elements of ''U'' ∩ ''W''. Then v and w belong to both ''U'' and ''W''. Because ''U'' is a subspace, then v + w belongs to ''U''. Similarly, since ''W'' is a subspace, then v + w belongs to ''W''. Thus, v + w belongs to ''U'' ∩ ''W''. # Let v belong to ''U'' ∩ ''W'', and let ''c'' be a scalar. Then v belongs to both ''U'' and ''W''. Since ''U'' and ''W'' are subspaces, ''c''v belongs to both ''U'' and ''W''. # Since ''U'' and ''W'' are vector spaces, then 0 belongs to both sets. Thus, 0 belongs to ''U'' ∩ ''W''. For every vector space ''V'', the set and ''V'' itself are subspaces of ''V''.

## Sum

If ''U'' and ''W'' are subspaces, their sum is the subspace : $U + W = \left\.$Vector space related operators. For example, the sum of two lines is the plane that contains them both. The dimension of the sum satisfies the inequality : $\max\left(\dim U,\dim W\right) \leq \dim\left(U + W\right) \leq \dim\left(U\right) + \dim\left(W\right).$ Here, the minimum only occurs if one subspace is contained in the other, while the maximum is the most general case. The dimension of the intersection and the sum are related by the following equation: : $\dim\left(U+W\right) = \dim\left(U\right) + \dim\left(W\right) - \dim\left(U \cap W\right).$ A set of subspaces is independent when the only intersection between any pair of subspaces is the trivial subspace. The
direct sum The direct sum is an operation from abstract algebra In algebra, which is a broad division of mathematics, abstract algebra (occasionally called modern algebra) is the study of algebraic structures. Algebraic structures include group (mathema ...
is the sum of independent subspaces, written as $U \oplus W$. An equivalent restatement is that a direct sum is a subspace sum under the condition that every subspace contributes to the span of the sum. The dimension of a direct sum $U \oplus W$ is the same as the sum of subspaces, but may be shortened because the dimension of the trivial subspace is zero. $\dim (U \oplus W) = \dim (U) + \dim (W)$

## Lattice of subspaces

The operations
intersection In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities ...
and sum make the set of all subspaces a bounded
modular lattice In the branch of mathematics called order theory Order theory is a branch of mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), ...
, where the subspace, the
least element In mathematics, especially in order theory, the greatest element of a subset S of a partially ordered set (poset) is an element of S that is greater than every other element of S. The term least element is defined duality (order theory), dually, ...
, is an
identity element In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and th ...
of the sum operation, and the identical subspace ''V'', the greatest element, is an identity element of the intersection operation.

## Orthogonal complements

If $V$ is an
inner product space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
and $N$ is a subset of $V$, then the
orthogonal complement In the mathematical Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities an ...
of $N$, denoted $N^$, is again a subspace. If $V$ is finite-dimensional and $N$ is a subspace, then the dimensions of $N$ and $N^$ satisfy the complementary relationship $\dim \left(N\right) + \dim \left(N^\right) = \dim \left(V\right)$. Moreover, no vector is orthogonal to itself, so $N \cap N^\perp = \$ and $V$ is the
direct sum The direct sum is an operation from abstract algebra In algebra, which is a broad division of mathematics, abstract algebra (occasionally called modern algebra) is the study of algebraic structures. Algebraic structures include group (mathema ...
of $N$ and $N^$. Applying orthogonal complements twice returns the original subspace: $\left(N^\right)^ = N$ for every subspace $N$. p. 195, § 6.51 This operation, understood as
negation In logic Logic is an interdisciplinary field which studies truth and reasoning Reason is the capacity of consciously making sense of things, applying logic Logic (from Ancient Greek, Greek: grc, wikt:λογική, λογική, ...

($\neg$), makes the lattice of subspaces a (possibly
infinite Infinite may refer to: Mathematics *Infinite set, a set that is not a finite set *Infinity, an abstract concept describing something without any limit Music *Infinite (band), a South Korean boy band *''Infinite'' (EP), debut EP of American mus ...
) orthocomplemented lattice (although not a distributive lattice). In spaces with other
bilinear form In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and th ...
s, some but not all of these results still hold. In
pseudo-Euclidean spaceIn mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ha ...
s and
symplectic vector space In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and the ...
s, for example, orthogonal complements exist. However, these spaces may have
null vector In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and quantities and ...
s that are orthogonal to themselves, and consequently there exist subspaces $N$ such that $N \cap N^ \ne \$. As a result, this operation does not turn the lattice of subspaces into a Boolean algebra (nor a
Heyting algebra __notoc__ Arend Heyting (; 9 May 1898 – 9 July 1980) was a Dutch Dutch commonly refers to: * Something of, from, or related to the Netherlands * Dutch people () * Dutch language () *Dutch language , spoken in Belgium (also referred as ''fl ...
).

# Algorithms

Most algorithms for dealing with subspaces involve row reduction. This is the process of applying
elementary row operation In mathematics, an elementary matrix is a matrix (mathematics), matrix which differs from the identity matrix by one single elementary row operation. The elementary matrices generate the general linear group GL''n''(F) when F is a field. Left multip ...
s to a matrix, until it reaches either
row echelon form In linear algebra, a Matrix (mathematics), matrix is in echelon form if it has the shape resulting from a Gaussian elimination. A matrix being in row echelon form means that Gaussian elimination has operated on the rows, and column echelon form ...
or
reduced row echelon form In linear algebra, a Matrix (mathematics), matrix is in echelon form if it has the shape resulting from a Gaussian elimination. A matrix being in row echelon form means that Gaussian elimination has operated on the rows, and column echelon form ...
. Row reduction has the following important properties: # The reduced matrix has the same null space as the original. # Row reduction does not change the span of the row vectors, i.e. the reduced matrix has the same row space as the original. # Row reduction does not affect the linear dependence of the column vectors.

## Basis for a row space

:Input An ''m'' × ''n'' matrix ''A''. :Output A basis for the row space of ''A''. :# Use elementary row operations to put ''A'' into row echelon form. :# The nonzero rows of the echelon form are a basis for the row space of ''A''. See the article on
row space In linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces a ...
for an
example Example may refer to: * ''exempli gratia Notes and references Notes References Sources * * * Further reading * * {{Latin phrases E ...'' (e.g.), usually read out in English as "for example" * .example, reserved as a domain na ...
. If we instead put the matrix ''A'' into reduced row echelon form, then the resulting basis for the row space is uniquely determined. This provides an algorithm for checking whether two row spaces are equal and, by extension, whether two subspaces of ''K''''n'' are equal.

## Subspace membership

:Input A basis for a subspace ''S'' of ''K''''n'', and a vector v with ''n'' components. :Output Determines whether v is an element of ''S'' :# Create a (''k'' + 1) × ''n'' matrix ''A'' whose rows are the vectors b1, ... , b''k'' and v. :# Use elementary row operations to put ''A'' into row echelon form. :# If the echelon form has a row of zeroes, then the vectors are linearly dependent, and therefore .

## Basis for a column space

:Input An ''m'' × ''n'' matrix ''A'' :Output A basis for the column space of ''A'' :# Use elementary row operations to put ''A'' into row echelon form. :# Determine which columns of the echelon form have pivots. The corresponding columns of the original matrix are a basis for the column space. See the article on column space for an
example Example may refer to: * ''exempli gratia Notes and references Notes References Sources * * * Further reading * * {{Latin phrases E ...'' (e.g.), usually read out in English as "for example" * .example, reserved as a domain na ...

. This produces a basis for the column space that is a subset of the original column vectors. It works because the columns with pivots are a basis for the column space of the echelon form, and row reduction does not change the linear dependence relationships between the columns.

## Coordinates for a vector

:Input A basis for a subspace ''S'' of ''K''''n'', and a vector :Output Numbers ''t''1, ''t''2, ..., ''t''''k'' such that :# Create an
augmented matrix Augment or augmentation may refer to: Language * Augment (Indo-European), a syllable added to the beginning of the word in certain Indo-European languages * Augment (Bantu languages), a morpheme that is prefixed to the noun class prefix of nouns ...
''A'' whose columns are b1,...,b''k'' , with the last column being v. :# Use elementary row operations to put ''A'' into reduced row echelon form. :# Express the final column of the reduced echelon form as a linear combination of the first ''k'' columns. The coefficients used are the desired numbers . (These should be precisely the first ''k'' entries in the final column of the reduced echelon form.) If the final column of the reduced row echelon form contains a pivot, then the input vector v does not lie in ''S''.

## Basis for a null space

:Input An ''m'' × ''n'' matrix ''A''. :Output A basis for the null space of ''A'' :# Use elementary row operations to put ''A'' in reduced row echelon form. :# Using the reduced row echelon form, determine which of the variables are free. Write equations for the dependent variables in terms of the free variables. :# For each free variable ''xi'', choose a vector in the null space for which and the remaining free variables are zero. The resulting collection of vectors is a basis for the null space of ''A''. See the article on null space for an
example Example may refer to: * ''exempli gratia Notes and references Notes References Sources * * * Further reading * * {{Latin phrases E ...'' (e.g.), usually read out in English as "for example" * .example, reserved as a domain na ...

.

## Basis for the sum and intersection of two subspaces

Given two subspaces and of , a basis of the sum $U + W$ and the intersection $U \cap W$ can be calculated using the Zassenhaus algorithm

## Equations for a subspace

:Input A basis for a subspace ''S'' of ''K''''n'' :Output An (''n'' − ''k'') × ''n'' matrix whose null space is ''S''. :# Create a matrix ''A'' whose rows are . :# Use elementary row operations to put ''A'' into reduced row echelon form. :# Let be the columns of the reduced row echelon form. For each column without a pivot, write an equation expressing the column as a linear combination of the columns with pivots. :# This results in a homogeneous system of ''n'' − ''k'' linear equations involving the variables c1,...,c''n''. The matrix corresponding to this system is the desired matrix with nullspace ''S''. ; Example :If the reduced row echelon form of ''A'' is :: :then the column vectors satisfy the equations ::$\begin \mathbf_3 &= -3\mathbf_1 + 5\mathbf_2 \\ \mathbf_5 &= 2\mathbf_1 - \mathbf_2 + 7\mathbf_4 \\ \mathbf_6 &= 4\mathbf_2 - 9\mathbf_4 \end$ :It follows that the row vectors of ''A'' satisfy the equations ::$\begin x_3 &= -3x_1 + 5x_2 \\ x_5 &= 2x_1 - x_2 + 7x_4 \\ x_6 &= 4x_2 - 9x_4. \end$ :In particular, the row vectors of ''A'' are a basis for the null space of the corresponding matrix.

* Cyclic subspace *
Invariant subspaceIn mathematics Mathematics (from Ancient Greek, Greek: ) includes the study of such topics as quantity (number theory), mathematical structure, structure (algebra), space (geometry), and calculus, change (mathematical analysis, analysis). It ha ...
*
Multilinear subspace learning Multilinear subspace learning is an approach to dimensionality reduction.M. A. O. Vasilescu, D. Terzopoulos (2003"Multilinear Subspace Analysis of Image Ensembles" "Proceedings of the IEEE Conference on Computer Vision and Pattern Recognition (CVP ...
*
Quotient space (linear algebra) In linear algebra Linear algebra is the branch of mathematics concerning linear equations such as: :a_1x_1+\cdots +a_nx_n=b, linear maps such as: :(x_1, \ldots, x_n) \mapsto a_1x_1+\cdots +a_nx_n, and their representations in vector spaces an ...
* Signal subspace *
Subspace topology In topology In mathematics Mathematics (from Greek: ) includes the study of such topics as numbers (arithmetic and number theory), formulas and related structures (algebra), shapes and spaces in which they are contained (geometry), and ...

# Sources

## Textbook

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