We have $A=\left(\begin{array}{cc}\omega & -\omega \\ 0 & \omega\end{array}\right)$, so
\[A^2 = A \cdot A = \left(\begin{array}{cc}\omega & -\omega \\ 0 & \omega\end{array}\right) \cdot \left(\begin{array}{cc}\omega & -\omega \\ 0 & \omega\end{array}\right) =
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