From this Wikipedia link.
Let $K$ be a field of characteristic zero and let $A$ be an $n \times n$ matrix over $K$. Prove the following:
(a) $N$ is nilpotent iff $\mathrm{tr}(N^m)=0$ for all $0<m \leq n$ iff $\mathrm{tr}(N^m)=0$ for all $m \in \mathbb{N_{+}}$.
(b) If $N$ is nilpotent, then $N$ is similar to a strictly upper triangular matrix.
Additional Query: Is above true if $K$ is not assumed to be algebraically closed? (Then one can't apply Jordan normal form anymore.)
Best Answer
The existence of a Jordan normal form of a (finite dimensional) vector space endomorphism $\phi$ does not depend on the fact that ground the field is algebraically closed, just that the minimal polynomial of $\phi$ splits over the ground field. Since an endomorphism is nilpotent if and only if its minimal polynomial is of the form $X^k$ for some $k\in\mathbf N$, nilpotent matrices always have a Jordan normal form.
This gives you point (b). Point (a) is a consequence of the fact that in characterestic $0$ the coefficients of the characteristic polynomial of $\phi$ can be expressed in terms of the values $\operatorname{tr}(\phi^k)$ for $0<k\leq n)$ via Newton's identities; if all of those traces are $0$ then the characteristic polynomial is $X^n$ and $\phi$ nilpotent. Apart from its characteristic, this does not depend on the ground field.