# matrix unit

A matrix unit is a matrix (over some ring with $1$) whose entries are all $0$ except in one cell, where it is $1$.

For example, among the $3\times 2$ matrices,

 $\begin{pmatrix}1&0\\ 0&0\\ 0&0\end{pmatrix},\quad\begin{pmatrix}0&1\\ 0&0\\ 0&0\end{pmatrix},\quad\begin{pmatrix}0&0\\ 1&0\\ 0&0\end{pmatrix},\quad\begin{pmatrix}0&0\\ 0&1\\ 0&0\end{pmatrix},\quad\begin{pmatrix}0&0\\ 0&0\\ 1&0\end{pmatrix},\quad\begin{pmatrix}0&0\\ 0&0\\ 0&1\end{pmatrix}$

are the matrix units.

Let $A$ and $B$ be $m\times n$ and $p\times q$ matrices over $R$, and $U_{ij}$ an $n\times p$ matrix unit (over $R$). Then

1. 1.

$AU_{ij}$ is the $m\times p$ matrix whose $j$th column is the $i$th column of $A$, and $0$ everywhere else, and

2. 2.

$U_{ij}B$ is the $n\times q$ matrix whose $i$th row is the $j$th row of $B$ and $0$ everywhere else.

Remarks. Let $M=M_{m\times n}(R)$ be the set of all $m$ by $n$ matrices with entries in a ring $R$ (with $1$). Denote $U_{ij}$ the matrix unit in $M$ whose cell $(i,j)$ is $1$.

• $M$ is a (left or right) $R$-module generated by the $m\times n$ matrix units.

• When $m=n$, $M$ has the structure of an algebra over $R$. The matrix units have the following properties:

1. (a)

$U_{ij}U_{k\ell}=\delta_{jk}U_{i\ell}$, and

2. (b)

$U_{11}+\cdots+U_{nn}=I_{n}$,

where $\delta_{ij}$ is the Kronecker delta and $I_{n}$ is the identity matrix. Note that the $U_{ii}$ form a complete set of pairwise orthogonal idempotents, meaning $U_{ii}U_{ii}=U_{ii}$ and $U_{ii}U_{jj}=0$ if $i\neq j$.

• In general, in a matrix ring $S$ (consisting of, say, all $n\times n$ matrices), any set of $n$ matrices satisfying the two properties above is called a full set of matrix units of $S$.

• For example, if $\{U_{ij}\mid 1\leq i,j\leq 2\}$ is the set of $2\times 2$ matrix units over $\mathbb{R}$, then for any invertible matrix $T$, $\{TU_{ij}T^{-1}\mid 1\leq i,j\leq 2\}$ is a full set of matrix units.

• If we embed $R$ as a subring of $M_{n}(R)$, then $R$ is the centralizer of the matrix units of $M_{n}(R)$, meaning that the only elements in $M_{n}(R)$ that commute with the matrix units are the elements in $R$.

## References

• 1 T. Y. Lam, Lectures on Modules and Rings, Springer, New York, 1998.
Title matrix unit MatrixUnit 2013-03-22 18:30:35 2013-03-22 18:30:35 CWoo (3771) CWoo (3771) 8 CWoo (3771) Definition msc 15A30 msc 16S50 ElementaryMatrix full set of matrix units