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\begin{equation*}
  z = \begin{bmatrix} z_1 \\ z_2 \end{bmatrix} \qquad
  A = \begin{bmatrix} a_{11} & a_{12} \\ 0 & a_{22} \end{bmatrix} \qquad
  B = \begin{bmatrix} 0 \\ 1 \end{bmatrix} \qquad
  C = \begin{bmatrix} 1 & 0 \end{bmatrix}
\end{equation*}
\end{document}