# Inner derivation implies endomorphism for class two Lie ring

## Statement

Suppose $L$ is a Lie ring of nilpotency class two (?). In other words, $[[x,y],z] = 0$ for all $x,y,z \in L$. Then, any inner derivation of $L$ is an endomorphism of $L$ as a Lie ring.

## Proof

Given: A Lie ring $L$ of nilpotency class two, an element $x \in L$.

To prove: The map $y \mapsto [x,y]$ is an endomorphism of $L$ as a Lie ring.

Proof: The map is clearly an endomorphism of the additive group of $L$, so it suffices to show that it preserves the Lie bracket. In other words, we need to show that for $y, z \in L$, we have:

$\! [x,[y,z]] = [[x,y],[x,z]]$.

But since $L$ has nilpotency class two, both sides are zero, so the result holds.