  Jacobson proved that if a Lie algebra admits an invertible derivation, it
must be nilpotent. He also suspected, though incorrectly, that the converse
might be true: that every nilpotent Lie algebra has an invertible derivation.
We prove that a Lie algebra is nilpotent if and only if it admits an invertible
Leibniz-derivation. The proofs are elementary in nature and are based on
well-known techniques.
