Consider the linear system $A \mathbf{x}=\mathbf{e}_1$ based on the $10 \times 10$ pentadiagonal matrix
$$
A=\left(\begin{array}{rrrrrrr}
z & -1 & 1 & 0 & & & \\
-1 & z & -1 & 1 & 0 & & \\
1 & -1 & z & -1 & 1 & 0 & \\
0 & 1 & -1 & z & -1 & 1 & \ddots \\
& 0 & 1 & -1 & z & -1 & \ddots \\
& & 0 & 1 & -1 & z & \ddots
\end{array}\right)
$$
(a) For what values of $z$ are the Jacobi and Gauss-Seidel Methods guaranteed to converge? (b) Set $z=4$. How many iterations are required to approximate the solution to 3 decimal places? (c) How small can $|z|$ be before the methods diverge?