###### Definition 3.1.2

We say that a matrix \(A\) is *lower triangular* if all its entries above the diagaonal are zero. Similarly, \(A\) is *upper triangular* if all the entries below the diagonal are zero.

We say that a matrix \(A\) is *lower triangular* if all its entries above the diagaonal are zero. Similarly, \(A\) is *upper triangular* if all the entries below the diagonal are zero.