Matrix powers are not injective: $A^2=(-A)^2$ can hold with $A\ne -A$.Suggest a slogan
For square matrices \(A,B\) and \(n\ge2\), \(A^n=B^n\) does not imply \(A=B\). In \(\Mat_2(\RR)\), let \[ A=\begin{bmatrix}0&1\\-1&0\end{bmatrix}. \] Then \[ A^2=\begin{bmatrix}-1&0\\0&-1\end{bmatrix}=(-A)^2, \] but \(A\ne-A\).
For matrices, does $A^n=B^n\implies A=B$?