This is LaTeX code:

[latex]
 
\left( \begin{array}{ccccccccc}
a & 0 & b & 0 & \ldots & 0 & 0 & 0 \\
0 & a & 0 & b & \ldots & 0 & 0 & 0 \\
0 & 0 & a & 0 & \ldots & 0 & 0 & 0 \\
\ldots & \ldots & \ldots & \ldots & \ldots & \ldots &
\ldots & \ldots \\
0 & 0 & 0 & 0 & \ldots & a & 0 & b \\
b & 0 & 0 & 0 & \ldots & 0 & a & 0 \\
0 & b & 0 & 0 & \ldots & 0 & 0 & a \\
\end{array} \right)
 
[/latex]