## Differential Equations and Linear Algebra (4th Edition)

$\begin{bmatrix} 1 & 3 & 4 & -3\\ 0 & 2 & -4 & 4\\ 0 & 0 & 5 & -2 \\ 0 & 0 & 0 & -1 \\ \end{bmatrix}$
Make $a_{ij}=0$, if $i\gt j$, that is: $a_{21}=a_{31}=a_{32}=a_{41}=a_{42}=a_{43}=0$ If $i\leq j$, $a_{ij}$ can assume any value. Example: $\begin{bmatrix} 1 & 3 & 4 & -3\\ 0 & 2 & -4 & 4\\ 0 & 0 & 5 & -2 \\ 0 & 0 & 0 & -1 \\ \end{bmatrix}$