TITLE: Is there a matrix $A$ such that: $A^4=\begin{pmatrix}0 & 2 & -1 & 1\\ 0 & 0 & 3 &1\\ 0 & 0& 0 & 4\\ 0 & 0 & 0 & 0 \\ \end{pmatrix} ~?$
QUESTION [5 upvotes]: Is there a matrix $A\in \mathbb{C}^{4\times 4}$ such that: $$A^4=\left(\begin{array}{cccc}
0 & 2 & -1 & 1\\
0 &  0 & 3 &1\\
0 & 0& 0 & 4\\
0 & 0 & 0 & 0 \\
\end{array} \right) ~?$$

Any hint is appreciated. Thanks a lot!

REPLY [11 votes]: No. Suppose $A$ is such a matrix. Then, $(A^4)^4=A^{16}=0$ so the minimal polynomial $m(x)$ of $A$ divides $x^{16}$. Since $A$ is $4\times 4$, its minimal polynomial is of degree $\leq 4$ so $m(x)$ divides $x^4$. Then, $A^4=0$, contradiction.
Added: The underlying idea behind this proof is that if an $n\times n$ matrix is nilpotent (i.e. $A^m=0$ for some $m>0$), then in fact $A^n=0$.

REPLY [2 votes]: If an $n\times n$  matrix satisfies $A^m=0$ for some $m$ then it satisfies $A^n=0$.
notice that if $A^4$ was equal to that matrix then $(A^4)^4=0$, but then $A^4=0$.