Every vector
a in three dimensions is a
linear combination of the standard basis vectors
i,
j, and
k.
In mathematics, the standard basis (also called natural basis or canonical basis) for a Euclidean space is the set of unit vectors pointing in the direction of the axes of a Cartesian coordinate system. For example, the standard basis for the Euclidean plane is formed by vectors

\mathbf{e}_x = (1,0),\quad \mathbf{e}_y = (0,1),
and the standard basis for threedimensional space is formed by vectors

\mathbf{e}_x = (1,0,0),\quad \mathbf{e}_y = (0,1,0),\quad \mathbf{e}_z=(0,0,1).
Here the vector e_{x} points in the x direction, the vector e_{y} points in the y direction, and the vector e_{z} points in the z direction. There are several common notations for these vectors, including {e_{x}, e_{y}, e_{z}}, {e_{1}, e_{2}, e_{3}}, {i, j, k}, and {x, y, z}. These vectors are sometimes written with a hat to emphasize their status as unit vectors. Each of these vectors is sometimes referred to as the versor of the corresponding Cartesian axis.
These vectors are a basis in the sense that any other vector can be expressed uniquely as a linear combination of these. For example, every vector v in threedimensional space can be written uniquely as

v_x\,\mathbf{e}_x + v_y\,\mathbf{e}_y + v_z\,\mathbf{e}_z,
the scalars v_{x}, v_{y}, v_{z} being the scalar components of the vector v.
In ndimensional Euclidean space, the standard basis consists of n distinct vectors

\{ \mathbf{e}_i : 1\leq i\leq n\},
where e_{i} denotes the vector with a 1 in the ith coordinate and 0's elsewhere.
Standard bases can be defined for other vector spaces, such as polynomials and matrices. In both cases, the standard basis consists of the elements of the vector space such that all coefficients but one are 0 and the nonzero one is 1. For polynomials, the standard basis thus consists of the monomials and is commonly called monomial basis. For matrices \mathcal{M}_{m \times n}, the standard basis consists of the m×nmatrices with exactly one nonzero entry, which is 1. For example, the standard basis for 2×2 matrices is formed by the 4 matrices

\mathbf{e}_{11} = \begin{pmatrix} 1 & 0 \\ 0 & 0 \end{pmatrix},\quad \mathbf{e}_{12} = \begin{pmatrix} 0 & 1 \\ 0 & 0 \end{pmatrix},\quad \mathbf{e}_{21} = \begin{pmatrix} 0 & 0 \\ 1 & 0 \end{pmatrix},\quad \mathbf{e}_{22} = \begin{pmatrix} 0 & 0 \\ 0 & 1 \end{pmatrix}.
Contents

Properties 1

Generalizations 2

Other usages 3

See also 4

References 5
Properties
By definition, the standard basis is a sequence of orthogonal unit vectors. In other words, it is an ordered and orthonormal basis.
However, an ordered orthonormal basis is not necessarily a standard basis. For instance the two vectors representing a 30° rotation of the 2D standard basis described above, i.e.

v_1 = \left( {\sqrt 3 \over 2} , {1 \over 2} \right) \,

v_2 = \left( {1 \over 2} , {\sqrt 3 \over 2} \right) \,
are also orthogonal unit vectors, but they are not aligned with the axes of the Cartesian coordinate system, so the basis with these vectors does not meet the definition of standard basis.
Generalizations
There is a standard basis also for the ring of polynomials in n indeterminates over a field, namely the monomials.
All of the preceding are special cases of the family

{(e_i)}_{i\in I}= ( (\delta_{ij} )_{j \in I} )_{i \in I}
where I is any set and \delta_{ij} is the Kronecker delta, equal to zero whenever i≠j and equal to 1 if i=j. This family is the canonical basis of the Rmodule (free module)

R^{(I)}
of all families

f=(f_i)
from I into a ring R, which are zero except for a finite number of indices, if we interpret 1 as 1_{R}, the unit in R.
Other usages
The existence of other 'standard' bases has become a topic of interest in algebraic geometry, beginning with work of Hodge from 1943 on Grassmannians. It is now a part of representation theory called standard monomial theory. The idea of standard basis in the universal enveloping algebra of a Lie algebra is established by the Poincaré–Birkhoff–Witt theorem.
Gröbner bases are also sometimes called standard bases.
In physics, the standard basis vectors for a given Euclidean space are sometimes referred to as the versors of the axes of the corresponding Cartesian coordinate system.
See also
References

Ryan, Patrick J. (1986). Euclidean and nonEuclidean geometry: an analytical approach. Cambridge; New York: Cambridge University Press. (page 198)

Schneider, Philip J.; Eberly, David H. (2003). Geometric tools for computer graphics. Amsterdam; Boston: Morgan Kaufmann Publishers. (page 112)
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