Here we will concentrate mostly on computational patterns involving diagonal matrices. (Computations involving upper triangular or lower triangular matrices are somewhat similar — see further below.)
Example 7.4.1. Matrix operations involving diagonal matrices.
Let's look at each of a sum, product, power, and inverse involving diagonal matrices, in the \(3\times 3\) case.
\begin{equation*}
\begin{bmatrix} 1 \amp 0 \amp 0 \\ 0 \amp 2 \amp 0 \\ 0 \amp 0 \amp 3 \end{bmatrix}
+ \left[\begin{array}{rrr} 4 \amp 0 \amp 0 \\ 0 \amp -2 \amp 0 \\ 0 \amp 0 \amp 6 \end{array}\right]
= \begin{bmatrix} 5 \amp 0 \amp 0 \\ 0 \amp 0 \amp 0 \\ 0 \amp 0 \amp 9 \end{bmatrix}
\end{equation*}
\begin{equation*}
\begin{bmatrix} 1 \amp 0 \amp 0 \\ 0 \amp 2 \amp 0 \\ 0 \amp 0 \amp 3 \end{bmatrix}
\left[\begin{array}{rrr} 4 \amp 0 \amp 0 \\ 0 \amp -2 \amp 0 \\ 0 \amp 0 \amp 6 \end{array}\right]
= \left[\begin{array}{rrr}
4 \amp 0 \amp 0 \\
0 \amp -4 \amp 0 \\
0 \amp 0 \amp 18
\end{array}\right]
\end{equation*}
\begin{equation*}
\begin{bmatrix} 1 \amp 0 \amp 0 \\ 0 \amp 2 \amp 0 \\ 0 \amp 0 \amp 3 \end{bmatrix}^2
= \begin{bmatrix} 1 \amp 0 \amp 0 \\ 0 \amp 4 \amp 0 \\ 0 \amp 0 \amp 9 \end{bmatrix}
= \begin{bmatrix} 1^2 \amp 0 \amp 0 \\ 0 \amp 2^2 \amp 0 \\ 0 \amp 0 \amp 3^2 \end{bmatrix}
\end{equation*}
\begin{equation*}
\inv{\begin{bmatrix} 1 \amp 0 \amp 0 \\ 0 \amp 2 \amp 0 \\ 0 \amp 0 \amp 3 \end{bmatrix}}
= \begin{bmatrix} 1 \amp 0 \amp 0 \\ 0 \amp \frac{1}{2} \amp 0 \\ 0 \amp 0 \amp \frac{1}{3} \end{bmatrix}
\end{equation*}
We can easily identify some patterns in the above example.
We have some of the same patterns for upper and lower triangular matrices, at least for the diagonal entries. We'll demonstrate with some upper triangular example computations.
Example 7.4.2. Basic matrix operations involving upper triangular matrices.
\begin{equation*}
\begin{bmatrix} 1 \amp 1 \amp 1 \\ 0 \amp 2 \amp 1 \\ 0 \amp 0 \amp 3 \end{bmatrix}
+
\left[\begin{array}{rrr} 4 \amp 1 \amp 1 \\ 0 \amp -2 \amp 1 \\ 0 \amp 0 \amp 6 \end{array}\right]
= \begin{bmatrix}
5 \amp 2 \amp 2 \\
0 \amp 0 \amp 2 \\
0 \amp 0 \amp 9
\end{bmatrix}
\end{equation*}
\begin{align*}
\begin{bmatrix} 1 \amp 1 \amp 1 \\ 0 \amp 2 \amp 1 \\ 0 \amp 0 \amp 3 \end{bmatrix}
\left[\begin{array}{rrr} 4 \amp 1 \amp 1 \\ 0 \amp -2 \amp 1 \\ 0 \amp 0 \amp 6 \end{array}\right]
\amp = \begin{bmatrix}
4+0+0 \amp 1-2+0 \amp 1+1+0 \\
0+0+0 \amp 0-4+0 \amp 0+2+6 \\
0+0+0 \amp 0+0+0 \amp 0+0+18
\end{bmatrix}\\
\amp = \left[\begin{array}{rrr}
4 \amp -1 \amp 2 \\
0 \amp -4 \amp 8 \\
0 \amp 0 \amp 18
\end{array}\right]
\end{align*}
\begin{align*}
\begin{bmatrix} 1 \amp 1 \amp 1 \\ 0 \amp 2 \amp 1 \\ 0 \amp 0 \amp 3 \end{bmatrix}^2
\amp = \begin{bmatrix} 1 \amp 1 \amp 1 \\ 0 \amp 2 \amp 1 \\ 0 \amp 0 \amp 3 \end{bmatrix}
\begin{bmatrix} 1 \amp 1 \amp 1 \\ 0 \amp 2 \amp 1 \\ 0 \amp 0 \amp 3 \end{bmatrix}\\
\amp = \begin{bmatrix}
1+0+0 \amp 1+2+0 \amp 1+1+3 \\
0+0+0 \amp 0+4+0 \amp 0+2+3 \\
0+0+0 \amp 0+0+0 \amp 0+0+9
\end{bmatrix}\\
\amp = \begin{bmatrix} 1 \amp 3 \amp 5 \\ 0 \amp 4 \amp 5 \\ 0 \amp 0 \amp 9 \end{bmatrix}\\
\amp = \begin{bmatrix} 1^2 \amp 3 \amp 5 \\ 0 \amp 2^2 \amp 5 \\ 0 \amp 0 \amp 3^2 \end{bmatrix}
\end{align*}
Computing the inverse of an upper triangular matrix is not as simple as for a diagonal matrix — some row reduction will be required, using Procedure 6.3.7.
Example 7.4.3. Inverse of an upper triangular matrix.
Augment with the identity and reduce.
\begin{align*}
\left[\begin{array}{rrr|rrr}
1 \amp 1 \amp 1 \amp 1 \amp 0 \amp 0 \\
0 \amp 2 \amp 2 \amp 0 \amp 1 \amp 0 \\
0 \amp 0 \amp 3 \amp 0 \amp 0 \amp 1
\end{array}\right]
\begin{matrix} \phantom{x} \\ \frac{1}{2}R_2 \\ \frac{1}{3}R_3 \end{matrix}
\longrightarrow
\left[\begin{array}{rrr|rrr}
1 \amp 1 \amp 1 \amp 1 \amp 0 \amp 0 \\
0 \amp 1 \amp 1 \amp 0 \amp \frac{1}{2} \amp 0 \\
0 \amp 0 \amp 1 \amp 0 \amp 0 \amp \frac{1}{3}
\end{array}\right]
\begin{matrix} R_1-R_2 \\ \phantom{x} \\ \phantom{x} \end{matrix}\\
\longrightarrow
\left[\begin{array}{rrr|rrr}
1 \amp 0 \amp 0 \amp 1 \amp -\frac{1}{2} \amp 0 \\
0 \amp 1 \amp 1 \amp 0 \amp \frac{1}{2} \amp 0 \\
0 \amp 0 \amp 1 \amp 0 \amp 0 \amp \frac{1}{3}
\end{array}\right]
\begin{matrix} \phantom{x} \\ R_2-R_3 \\ \phantom{x} \end{matrix}
\longrightarrow
\left[\begin{array}{rrr|rrr}
1 \amp 0 \amp 0 \amp 1 \amp -\frac{1}{2} \amp 0 \\
0 \amp 1 \amp 0 \amp 0 \amp \frac{1}{2} \amp -\frac{1}{3} \\
0 \amp 0 \amp 1 \amp 0 \amp 0 \amp \frac{1}{3}
\end{array}\right]
\end{align*}
With this reduction, we have calculated that
\begin{equation*}
\inv{\begin{bmatrix}
1 \amp 1 \amp 1 \\
0 \amp 2 \amp 2 \\
0 \amp 0 \amp 3
\end{bmatrix}} =
\left[\begin{array}{rrr}
1 \amp -\frac{1}{2} \amp 0 \\
0 \amp \frac{1}{2} \amp -\frac{1}{3} \\
0 \amp 0 \amp \frac{1}{3}
\end{array}\right].
\end{equation*}
Again, in these two examples we see the same patterns on the main diagonal as for diagonal matrices. Products, powers, and inverses of lower triangular matrices would be similar.