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If $\mathrm{A}=\left(\begin{array}{cccc}1 & 0 & 0 & 1 \\ 0 & -1 & 0 & -1 \\ 0 & 0 & i & i \\ 0 & 0 & 0 & -i\end{array}\right)$

the matrix $\mathrm{A}^{4}$, calculated by the use of Cayley - Hamilton theorem or otherwise, is