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 matric ...

, a matrix unit is a

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** '' The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchi ...

with only one nonzero entry with value 1. The matrix unit with a 1 in the ''i''th row and ''j''th column is denoted as

E_

. For example, the 3 by 3 matrix unit with ''i'' = 1 and ''j'' = 2 is

E_ = \begin0 & 1 & 0 \\0 & 0 & 0 \\ 0 & 0 & 0 \end

A vector unit is a

standard unit vector In mathematics, the standard basis (also called natural basis or canonical basis) of a coordinate vector space (such as \mathbb^n or \mathbb^n) is the set of vectors whose components are all zero, except one that equals 1. For example, in th ...

. A single-entry matrix generalizes the matrix unit for matrices with only one nonzero entry of any value, not necessarily of value 1.

Properties

The set of ''m'' by ''n'' matrix units is a basis of the space of ''m'' by ''n'' matrices. The product of two matrix units of the same square shape

n \times n

satisfies the relation

E_E_ = \delta_E_,

where

\delta_

is the

Kronecker delta In mathematics, the Kronecker delta (named after Leopold Kronecker) is a function of two variables, usually just non-negative integers. The function is 1 if the variables are equal, and 0 otherwise: \delta_ = \begin 0 &\text i \neq j, \\ 1 ...

. The group of scalar ''n''-by-''n'' matrices over a ring ''R'' is the

centralizer In mathematics, especially group theory, the centralizer (also called commutant) of a subset ''S'' in a group ''G'' is the set of elements \mathrm_G(S) of ''G'' such that each member g \in \mathrm_G(S) commutes with each element of ''S'', ...

of the subset of ''n''-by-''n'' matrix units in the set of ''n''-by-''n'' matrices over ''R''. When multiplied by another matrix, it isolates a specific row or column in arbitrary position. For example, for any 3-by-3 matrix ''A'': :

E_A = \left \begin 0 & 0& 0 \\ a_ & a_ & a_ \\ 0 & 0 & 0 \end\right

AE_ = \left \begin 0 & 0 & a_ \\ 0 & 0 & a_ \\ 0 & 0 & a_  \end\right

References

Sparse matrices 1 (number) {{Linear-algebra-stub