Basis (linear algebra)

File:3d two bases same vector.svg

The same vector (dark purple) can be represented in two different bases (purple and red arrows).

In mathematics, a set Template:Mvar of elements of a vector space V is called a basis (Template:Plural form: bases) if every element of V can be written in a unique way as a finite linear combination of elements of Template:Mvar. The coefficients of this linear combination are referred to as components or coordinates of the vector with respect to Template:Mvar. The elements of a basis are called Template:Visible anchor.

Equivalently, a set Template:Mvar is a basis if its elements are linearly independent and every element of Template:Mvar is a linear combination of elements of Template:Mvar. In other words, a basis is a linearly independent spanning set.

A vector space can have several bases; however all the bases have the same number of elements, called the dimension of the vector space.

This article deals mainly with finite-dimensional vector spaces. However, many of the principles are also valid for infinite-dimensional vector spaces.

Basis vectors find applications in the study of crystal structures and frames of reference.

Definition

A basis B of a vector space V over a field F (such as the real numbers R or the complex numbers C) is a linearly independent subset of V that spans V. This means that a subset Template:Mvar of V is a basis if it satisfies the two following conditions:Template:R

• linear independence: for every finite subset $\{{\mathbf{𝐯}}_1,\dots ,{\mathbf{𝐯}}_m\}$ of Template:Mvar, if $c_1{\mathbf{𝐯}}_1+\cdots +c_m{\mathbf{𝐯}}_m=\mathbf{𝟎}$ for some $c_1,\dots ,c_m$ in Template:Mvar, then $c_1=\cdots =c_m=0$; andTemplate:R
• the spanning property: for every vector $\mathbf{𝐯}$ in $V$, one can choose $a_1,\dots ,a_n$ in $F$ and ${\mathbf{𝐯}}_1,\dots ,{\mathbf{𝐯}}_n$ in $B$ such that $\mathbf{𝐯}=a_1{\mathbf{𝐯}}_1+\cdots +a_n{\mathbf{𝐯}}_n$. In other words, $\mathbb{𝕧}$ can be represented as a linear combination of some vectors in $B$.Template:R

The first property may be equivalently phrased as follows: if a linear combination of vectors in Template:Mvar is equal to the zero vector, then all the scalars coefficients of the combination must be zero.

If Template:Mvar is a basis for Template:Mvar, then every vector $\mathbf{𝐯}$ in Template:Mvar can be written as a linear combination of vectors in Template:Mvar (by the spanning property); it follows from linear independence that this can be done in exactly one way. Thus, the scalar coefficients $a_i$ that appear in this combination are uniquely determined; they are called the coordinates of $\mathbf{𝐯}$ with respect to the basis $B$.

A vector space that has a finite basis is called finite-dimensional. In this case, the finite subset can be taken as $B$ itself to check for linear independence in the above definition.Template:R

It is often convenient or even necessary to have an ordering on the basis vectors, for example, when discussing orientation, or when one considers the scalar coefficients of a vector with respect to a basis without referring explicitly to the basis elements. In this case, the ordering is necessary for associating each coefficient with the corresponding basis element. This ordering can be done by numbering the basis elements. In order to emphasize that an order has been chosen, one speaks of an ordered basis, which is therefore not simply an unstructured set, but a sequence, an indexed family, or similar; see Template:Slink below.

Examples

File:Basis graph (no label).svg

This picture illustrates the standard basis in R². The blue and orange vectors are the elements of the basis; the green vector can be given in terms of the basis vectors, and so is linearly dependent upon them.

The set R² of the ordered pairs of real numbers is a vector space under the operations of component-wise addition $$(a,b)+(c,d)=(a+c,b+d)$$ and scalar multiplication $$\lambda (a,b)=(\lambda a,\lambda b),$$ where $\lambda$ is any real number. A simple basis of this vector space consists of the two vectors e₁ = (1, 0) and e₂ = (0, 1). These vectors form a basis (called the standard basis) because any vector v = (a, b) of R² may be uniquely written as $$\mathbf{𝐯}=a{\mathbf{𝐞}}_1+b{\mathbf{𝐞}}_2.$$ Any other pair of linearly independent vectors of R², such as (1, 1) and (−1, 2), forms also a basis of R².

More generally, if Template:Mvar is a field, the set $F^n$ of [[tuple|Template:Mvar-tuples]] of elements of Template:Mvar is a vector space for similarly defined addition and scalar multiplication. Let $${\mathbf{𝐞}}_i=(0,\dots ,0,1,0,\dots ,0)$$ be the Template:Mvar-tuple with all components equal to 0, except the Template:Mvarth, which is 1. Then ${\mathbf{𝐞}}_1,\dots ,{\mathbf{𝐞}}_n$ is a basis of $F^n,$ which is called the standard basis of $F^n.$

A different flavor of example is given by polynomial rings. If Template:Mvar is a field, the collection F[X] of all polynomials in one indeterminate Template:Mvar with coefficients in Template:Mvar is an Template:Mvar-vector space. One basis for this space is the monomial basis Template:Mvar, consisting of all monomials: $$B=\{1,X,X^2,\dots \}.$$ Any set of polynomials such that there is exactly one polynomial of each degree (such as the Bernstein basis polynomials or Chebyshev polynomials) is also a basis. (Such a set of polynomials is called a polynomial sequence.) But there are also many bases for F[X] that are not of this form.

Properties

Many properties of finite bases result from the Steinitz exchange lemma, which states that, for any vector space Template:Mvar, given a finite spanning set Template:Mvar and a linearly independent set Template:Mvar of Template:Mvar elements of Template:Mvar, one may replace Template:Mvar well-chosen elements of Template:Mvar by the elements of Template:Mvar to get a spanning set containing Template:Mvar, having its other elements in Template:Mvar, and having the same number of elements as Template:Mvar.

Most properties resulting from the Steinitz exchange lemma remain true when there is no finite spanning set, but their proofs in the infinite case generally require the axiom of choice or a weaker form of it, such as the ultrafilter lemma.

If Template:Mvar is a vector space over a field Template:Mvar, then:

• If Template:Mvar is a linearly independent subset of a spanning set S ⊆ V, then there is a basis Template:Mvar such that $$L\subseteq B\subseteq S.$$
• Template:Mvar has a basis (this is the preceding property with Template:Mvar being the empty set, and S = V).
• All bases of Template:Mvar have the same cardinality, which is called the dimension of Template:Mvar. This is the dimension theorem.
• A generating set Template:Mvar is a basis of Template:Mvar if and only if it is minimal, that is, no proper subset of Template:Mvar is also a generating set of Template:Mvar.
• A linearly independent set Template:Mvar is a basis if and only if it is maximal, that is, it is not a proper subset of any linearly independent set.

If Template:Mvar is a vector space of dimension Template:Mvar, then:

• A subset of Template:Mvar with Template:Mvar elements is a basis if and only if it is linearly independent.
• A subset of Template:Mvar with Template:Mvar elements is a basis if and only if it is a spanning set of Template:Mvar.

Coordinates

Let Template:Mvar be a vector space of finite dimension Template:Mvar over a field Template:Mvar, and $$B=\{{\mathbf{𝐛}}_1,\dots ,{\mathbf{𝐛}}_n\}$$ be a basis of Template:Mvar. By definition of a basis, every v in Template:Mvar may be written, in a unique way, as $$\mathbf{𝐯}={\lambda }_1{\mathbf{𝐛}}_1+\cdots +{\lambda }_n{\mathbf{𝐛}}_n,$$ where the coefficients ${\lambda }_1,\dots ,{\lambda }_n$ are scalars (that is, elements of Template:Mvar), which are called the coordinates of v over Template:Mvar. However, if one talks of the set of the coefficients, one loses the correspondence between coefficients and basis elements, and several vectors may have the same set of coefficients. For example, $3{\mathbf{𝐛}}_1+2{\mathbf{𝐛}}_2$ and $2{\mathbf{𝐛}}_1+3{\mathbf{𝐛}}_2$ have the same set of coefficients {2, 3}, and are different. It is therefore often convenient to work with an ordered basis; this is typically done by indexing the basis elements by the first natural numbers. Then, the coordinates of a vector form a sequence similarly indexed, and a vector is completely characterized by the sequence of coordinates. An ordered basis, especially when used in conjunction with an origin, is also called a coordinate frame or simply a frame (for example, a Cartesian frame or an affine frame).

Let, as usual, $F^n$ be the set of the [[tuple|Template:Mvar-tuples]] of elements of Template:Mvar. This set is an Template:Mvar-vector space, with addition and scalar multiplication defined component-wise. The map $$\phi :({\lambda }_1,\dots ,{\lambda }_n)\mapsto {\lambda }_1{\mathbf{𝐛}}_1+\cdots +{\lambda }_n{\mathbf{𝐛}}_n$$ is a linear isomorphism from the vector space $F^n$ onto Template:Mvar. In other words, $F^n$ is the coordinate space of Template:Mvar, and the Template:Mvar-tuple ${\phi }^{-1}(\mathbf{𝐯})$ is the coordinate vector of v.

The inverse image by $\phi$ of ${\mathbf{𝐛}}_i$ is the Template:Mvar-tuple ${\mathbf{𝐞}}_i$ all of whose components are 0, except the Template:Mvarth that is 1. The ${\mathbf{𝐞}}_i$ form an ordered basis of $F^n$, which is called its standard basis or canonical basis. The ordered basis Template:Mvar is the image by $\phi$ of the canonical basis of $F^n$.

It follows from what precedes that every ordered basis is the image by a linear isomorphism of the canonical basis of $F^n$, and that every linear isomorphism from $F^n$ onto Template:Mvar may be defined as the isomorphism that maps the canonical basis of $F^n$ onto a given ordered basis of Template:Mvar. In other words, it is equivalent to define an ordered basis of Template:Mvar, or a linear isomorphism from $F^n$ onto Template:Mvar.

Change of basis

Let V be a vector space of dimension Template:Mvar over a field F. Given two (ordered) bases $B_{\text{old}}=({\mathbf{𝐯}}_1,\dots ,{\mathbf{𝐯}}_n)$ and $B_{\text{new}}=({\mathbf{𝐰}}_1,\dots ,{\mathbf{𝐰}}_n)$ of V, it is often useful to express the coordinates of a vector Template:Mvar with respect to $B_{\mathrm{o}\mathrm{l}\mathrm{d}}$ in terms of the coordinates with respect to $B_{\mathrm{n}\mathrm{e}\mathrm{w}}.$ This can be done by the change-of-basis formula, that is described below. The subscripts "old" and "new" have been chosen because it is customary to refer to $B_{\mathrm{o}\mathrm{l}\mathrm{d}}$ and $B_{\mathrm{n}\mathrm{e}\mathrm{w}}$ as the old basis and the new basis, respectively. It is useful to describe the old coordinates in terms of the new ones, because, in general, one has expressions involving the old coordinates, and if one wants to obtain equivalent expressions in terms of the new coordinates; this is obtained by replacing the old coordinates by their expressions in terms of the new coordinates.

Typically, the new basis vectors are given by their coordinates over the old basis, that is, $${\mathbf{𝐰}}_j=\sum _{i=1}^n{a}_{i,j}{\mathbf{𝐯}}_i.$$ If $(x_1,\dots ,x_n)$ and $(y_1,\dots ,y_n)$ are the coordinates of a vector x over the old and the new basis respectively, the change-of-basis formula is $$x_i=\sum _{j=1}^n{a}_{i,j}{y}_j,$$ for i = 1, ..., n.

This formula may be concisely written in matrix notation. Let Template:Mvar be the matrix of the $a_{i,j}$, and $$X=\left[\begin{array}{c}{x}_1\\ ⋮\\ x_n\end{array}\right]\text{and}Y=\left[\begin{array}{c}{y}_1\\ ⋮\\ y_n\end{array}\right]$$ be the column vectors of the coordinates of v in the old and the new basis respectively, then the formula for changing coordinates is $$X=AY.$$

The formula can be proven by considering the decomposition of the vector x on the two bases: one has $$\mathbf{𝐱}=\sum _{i=1}^n{x}_i{\mathbf{𝐯}}_i,$$ and $$\mathbf{𝐱}=\sum _{j=1}^n{y}_j{\mathbf{𝐰}}_j=\sum _{j=1}^n{y}_j\sum _{i=1}^n{a}_{i,j}{\mathbf{𝐯}}_i=\sum _{i=1}^n\left(\sum _{j=1}^n{a}_{i,j}{y}_j\right){\mathbf{𝐯}}_i.$$

The change-of-basis formula results then from the uniqueness of the decomposition of a vector over a basis, here $B_{\text{old}}$; that is $$x_i=\sum _{j=1}^n{a}_{i,j}{y}_j,$$ for i = 1, ..., n.

Related notions

Free module

If one replaces the field occurring in the definition of a vector space by a ring, one gets the definition of a module. For modules, linear independence and spanning sets are defined exactly as for vector spaces, although "generating set" is more commonly used than that of "spanning set".

Like for vector spaces, a basis of a module is a linearly independent subset that is also a generating set. A major difference with the theory of vector spaces is that not every module has a basis. A module that has a basis is called a free module. Free modules play a fundamental role in module theory, as they may be used for describing the structure of non-free modules through free resolutions.

A module over the integers is exactly the same thing as an abelian group. Thus a free module over the integers is also a free abelian group. Free abelian groups have specific properties that are not shared by modules over other rings. Specifically, every subgroup of a free abelian group is a free abelian group, and, if Template:Mvar is a subgroup of a finitely generated free abelian group Template:Mvar (that is an abelian group that has a finite basis), then there is a basis ${\mathbf{𝐞}}_1,\dots ,{\mathbf{𝐞}}_n$ of Template:Mvar and an integer 0 ≤ k ≤ n such that $a_1{\mathbf{𝐞}}_1,\dots ,a_k{\mathbf{𝐞}}_k$ is a basis of Template:Mvar, for some nonzero integers $a_1,\dots ,a_k$. For details, see Template:Slink.

Analysis

In the context of infinite-dimensional vector spaces over the real or complex numbers, the term Template:Visible anchor (named after Georg Hamel) or algebraic basis can be used to refer to a basis as defined in this article. This is to make a distinction with other notions of "basis" that exist when infinite-dimensional vector spaces are endowed with extra structure. The most important alternatives are orthogonal bases on Hilbert spaces, Schauder bases, and Markushevich bases on normed linear spaces. In the case of the real numbers R viewed as a vector space over the field Q of rational numbers, Hamel bases are uncountable, and have specifically the cardinality of the continuum, which is the cardinal number $2^{{\mathrm{\aleph }}_0}$, where ${\mathrm{\aleph }}_0$ (aleph-nought) is the smallest infinite cardinal, the cardinal of the integers.

The common feature of the other notions is that they permit the taking of infinite linear combinations of the basis vectors in order to generate the space. This, of course, requires that infinite sums are meaningfully defined on these spaces, as is the case for topological vector spaces – a large class of vector spaces including e.g. Hilbert spaces, Banach spaces, or Fréchet spaces.

The preference of other types of bases for infinite-dimensional spaces is justified by the fact that the Hamel basis becomes "too big" in Banach spaces: If X is an infinite-dimensional normed vector space that is complete (i.e. X is a Banach space), then any Hamel basis of X is necessarily uncountable. This is a consequence of the Baire category theorem. The completeness as well as infinite dimension are crucial assumptions in the previous claim. Indeed, finite-dimensional spaces have by definition finite bases and there are infinite-dimensional (non-complete) normed spaces that have countable Hamel bases. Consider $c_{00}$, the space of the sequences $x=(x_n)$ of real numbers that have only finitely many non-zero elements, with the norm $\Vert x\Vert =\underset{n}{\sup }|x_n|$. Its standard basis, consisting of the sequences having only one non-zero element, which is equal to 1, is a countable Hamel basis.

Example

In the study of Fourier series, one learns that the functions {1} ∪ { sin(nx), cos(nx) : n = 1, 2, 3, ... } are an "orthogonal basis" of the (real or complex) vector space of all (real or complex valued) functions on the interval [0, 2π] that are square-integrable on this interval, i.e., functions f satisfying $$\underset{0}{\overset{2\pi }{\int }}{|f(x)|}^{2}dx<\mathrm{\infty }.$$

The functions {1} ∪ { sin(nx), cos(nx) : n = 1, 2, 3, ... } are linearly independent, and every function f that is square-integrable on [0, 2π] is an "infinite linear combination" of them, in the sense that $$\underset{n\to \mathrm{\infty }}{\lim }\underset{0}{\overset{2\pi }{\int }}|a_0+\sum _{k=1}^n(a_k\cos \left(kx\right)+b_k\sin \left(kx\right))-f(x){|}^{2}dx=0$$

for suitable (real or complex) coefficients a_k, b_k. But many square-integrable functions cannot be represented as finite linear combinations of these basis functions, which therefore do not comprise a Hamel basis. Every Hamel basis of this space is much bigger than this merely countably infinite set of functions. Hamel bases of spaces of this kind are typically not useful, whereas orthonormal bases of these spaces are essential in Fourier analysis.

Geometry

The geometric notions of an affine space, projective space, convex set, and cone have related notions of basis. An affine basis for an n-dimensional affine space is $n+1$ points in general linear position. A Template:Visible anchor is $n+2$ points in general position, in a projective space of dimension n. A Template:Visible anchor of a polytope is the set of the vertices of its convex hull. A Template:Visible anchor consists of one point by edge of a polygonal cone. See also a Hilbert basis (linear programming).

Random basis

For a probability distribution in Rⁿ with a probability density function, such as the equidistribution in an n-dimensional ball with respect to Lebesgue measure, it can be shown that Template:Mvar randomly and independently chosen vectors will form a basis with probability one, which is due to the fact that Template:Mvar linearly dependent vectors x₁, ..., x_n in Rⁿ should satisfy the equation det[x₁ ⋯ x_n] = 0 (zero determinant of the matrix with columns x_i), and the set of zeros of a non-trivial polynomial has zero measure. This observation has led to techniques for approximating random bases.

File:Random almost orthogonal sets.png

Empirical distribution of lengths N of pairwise almost orthogonal chains of vectors that are independently randomly sampled from the n-dimensional cube [−1, 1]ⁿ as a function of dimension, n. Boxplots show the second and third quartiles of this data for each n, red bars correspond to the medians, and blue stars indicate means. Red curve shows theoretical bound given by Eq. (1) and green curve shows a refined estimate.

It is difficult to check numerically the linear dependence or exact orthogonality. Therefore, the notion of ε-orthogonality is used. For spaces with inner product, x is ε-orthogonal to y if $\left|⟨x,y⟩\right|/\left(\Vert x\Vert \Vert y\Vert \right)<\epsilon$ (that is, cosine of the angle between Template:Mvar and Template:Mvar is less than Template:Mvar).

In high dimensions, two independent random vectors are with high probability almost orthogonal, and the number of independent random vectors, which all are with given high probability pairwise almost orthogonal, grows exponentially with dimension. More precisely, consider equidistribution in n-dimensional ball. Choose N independent random vectors from a ball (they are independent and identically distributed). Let θ be a small positive number. Then for Template:NumBlk

Template:Mvar random vectors are all pairwise ε-orthogonal with probability 1 − θ. This Template:Mvar growth exponentially with dimension Template:Mvar and $N\gg n$ for sufficiently big Template:Mvar. This property of random bases is a manifestation of the so-called Template:Em.

The figure (right) illustrates distribution of lengths N of pairwise almost orthogonal chains of vectors that are independently randomly sampled from the n-dimensional cube [−1, 1]ⁿ as a function of dimension, n. A point is first randomly selected in the cube. The second point is randomly chosen in the same cube. If the angle between the vectors was within π/2 ± 0.037π/2 then the vector was retained. At the next step a new vector is generated in the same hypercube, and its angles with the previously generated vectors are evaluated. If these angles are within π/2 ± 0.037π/2 then the vector is retained. The process is repeated until the chain of almost orthogonality breaks, and the number of such pairwise almost orthogonal vectors (length of the chain) is recorded. For each n, 20 pairwise almost orthogonal chains were constructed numerically for each dimension. Distribution of the length of these chains is presented.

Proof that every vector space has a basis

Let V be any vector space over some field F. Let X be the set of all linearly independent subsets of V.

The set X is nonempty since the empty set is an independent subset of V, and it is partially ordered by inclusion, which is denoted, as usual, by ⊆.

Let Y be a subset of X that is totally ordered by ⊆, and let L_Y be the union of all the elements of Y (which are themselves certain subsets of V).

Since (Y, ⊆) is totally ordered, every finite subset of L_Y is a subset of an element of Y, which is a linearly independent subset of V, and hence L_Y is linearly independent. Thus L_Y is an element of X. Therefore, L_Y is an upper bound for Y in (X, ⊆): it is an element of X, that contains every element of Y.

As X is nonempty, and every totally ordered subset of (X, ⊆) has an upper bound in X, Zorn's lemma asserts that X has a maximal element. In other words, there exists some element L_max of X satisfying the condition that whenever L_max ⊆ L for some element L of X, then L = L_max.

It remains to prove that L_max is a basis of V. Since L_max belongs to X, we already know that L_max is a linearly independent subset of V.

If there were some vector w of V that is not in the span of L_max, then w would not be an element of L_max either. Let L_w = L_max ∪ {w}. This set is an element of X, that is, it is a linearly independent subset of V (because w is not in the span of L_max, and L_max is independent). As L_max ⊆ L_w, and L_max ≠ L_w (because L_w contains the vector w that is not contained in L_max), this contradicts the maximality of L_max. Thus this shows that L_max spans V.

Hence L_max is linearly independent and spans V. It is thus a basis of V, and this proves that every vector space has a basis.

This proof relies on Zorn's lemma, which is equivalent to the axiom of choice. Conversely, it has been proved that if every vector space has a basis, then the axiom of choice is true. Thus the two assertions are equivalent.

See also

• Basis of a matroid
• Basis of a linear program
• Coordinate system
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References

Historical references

• , reprint:

External links

• Instructional videos from Khan Academy
  • Introduction to bases of subspaces
  • Proof that any subspace basis has same number of elements
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• Template:Springer