Solution
= Solution
The exact solution obeys $(D-L)x=Ux+b$. Subtract this from the iteration to get $(D-L)e_n=Ue_{n-1}$. Hence the <stationary iteration matrix> is
$$
\boxed{G=(D-L)^{-1}U
=\begin{pmatrix}1/4&0\\-1/8&1/2\end{pmatrix}
\begin{pmatrix}0&-3\\0&0\end{pmatrix}
=\begin{pmatrix}0&-3/4\\0&3/8\end{pmatrix},\qquad e_n=Ge_{n-1}.}
$$
This is exactly the matrix $B$ from part (a); the sign follows the given $A=D-L-U$ convention.