TITLE: Nilpotent elements of a non-commutative ring with trivial automorphism group form an ideal
QUESTION [5 upvotes]: Let $R$ be a non-commutative ring with identity such that the identity map is the only ring automorphism of $R$. Prove that the set $N$ of all nilpotent elements of $R$ is an ideal of $R$.

REPLY [8 votes]: Hints:

In such a ring, every invertible element is central, else there is a nontrivial inner automorphism.
If $x$ is nilpotent, then $1-x$ is invertible.