Elementary matrices/Introduction/Section

Definition

Let ${}K$ be a field, and let ${}M$ be an ${}m\times n$-matrix over ${}K$. Then the following manipulations on ${}M$ are called elementary row operations.

1. Transpositions of two rows.
2. Multiplication of a row with a scalar ${}s\neq 0$.
3. Addition of ${}a$ times a row to another row.

Definition

Let ${}K$ be a field. We denote by ${}B_{ij}$ the ${}n\times n$-matrix with entry ${}1$ at the position ${}(i,j)$, and entry ${}0$ everywhere else. Then the following matrices are called elementary matrices.

1. ${}V_{ij}:=E_{n}-B_{ii}-B_{jj}+B_{ij}+B_{ji}$.
2. ${}S_{k}(s):=E_{n}+(s-1)B_{kk}{\text{ for }}s\neq 0$.
3. ${}A_{ij}(a):=E_{n}+aB_{ij}{\text{ for }}i\neq j{\text{ and }}a\in K$.

In detail, these elementary matrices look as follows.

${}V_{ij}={\begin{pmatrix}1&0&\cdots &\cdots &\cdots &\cdots &0\\\vdots &\ddots &\cdots &\cdots &\cdots &\cdots &\vdots \\0&\cdots &0&\cdots &1&\cdots &0\\\vdots &\cdots &\cdots &1&\cdots &\cdots &\vdots \\0&\cdots &1&\cdots &0&\cdots &0\\\vdots &\cdots &\cdots &\cdots &\cdots &\ddots &\vdots \\0&\cdots &\cdots &\cdots &\cdots &0&1\end{pmatrix}}\,.$

${}S_{k}(s)={\begin{pmatrix}1&0&\cdots &\cdots &\cdots &\cdots &0\\\vdots &\ddots &\ddots &\vdots &\vdots &\vdots &\vdots \\0&\cdots &1&0&\cdots &\cdots &0\\0&\cdots &0&s&0&\cdots &0\\0&\cdots &\cdots &0&1&\cdots &0\\\vdots &\vdots &\vdots &\vdots &\ddots &\ddots &\vdots \\0&\cdots &\cdots &\cdots &\cdots &0&1\end{pmatrix}}\,.$

${}A_{ij}(a)={\begin{pmatrix}1&0&\cdots &\cdots &\cdots &\cdots &0\\\vdots &\ddots &\cdots &\cdots &\cdots &\cdots &\vdots \\0&\cdots &1&\cdots &a&\cdots &0\\\vdots &\cdots &\cdots &\ddots &\cdots &\cdots &\vdots \\0&\cdots &\cdots &\cdots &1&\cdots &0\\\vdots &\cdots &\cdots &\cdots &\cdots &\ddots &\vdots \\0&\cdots &\cdots &\cdots &\cdots &0&1\end{pmatrix}}\,.$

Elementary matrices are invertible, see exercise.

Lemma

Let ${}K$ be a field and ${}M$ a ${}n\times n$-matrix with entries in ${}K$. Then the multiplication by elementary matrices from the left with ${}M$ has the following effects.

1. ${}V_{ij}\circ M=$ exchange of the ${}i$-th and the ${}j$-th row of ${}M$.
2. ${}(S_{k}(s))\circ M=$ multiplication of the ${}k$-th row of ${}M$ by ${}s$.
3. ${}(A_{ij}(a))\circ M=$ addition of ${}a$-times the ${}j$-th row of ${}M$ to the ${}i$-th row (${}i\neq j$).

Proof

See exercise.

$\Box$

Elementary row operations do not change the solution space of a homogeneous linear system, as shown in fact.