# Nilpotent ideal

A one- or two-sided ideal $M$ in a ring or semi-group with zero such that $M^n=\{0\}$ for some natural number $n$, that is, the product of any $n$ elements of $M$ vanishes. For example, in the residue class ring $\mathbf Z/p^n\mathbf Z$ modulo $p^n$, where $p$ is a prime number, every ideal except the ring itself is nilpotent. In the group ring $\mathbf F_p[G]$ of a finite $p$-group $G$ over the field with $p$ elements the ideal generated by the elements of the form $\sigma-1$, $\sigma\in G$, is nilpotent. In the ring of upper-triangular matrices over a field the matrices with 0's along the main diagonal form a nilpotent ideal.
All nilpotent ideals of a commutative ring are contained in the nil radical, which, in general, need not be a nilpotent but only a nil ideal. A simple example of this situation is the direct sum of the rings $\mathbf Z/p^n\mathbf Z$ for all natural numbers $n$. In a commutative ring every nilpotent element $a$ is contained in some nilpotent ideal, for example, in the principal ideal generated by $a$. In a non-commutative ring there may by nilpotent elements that are not contained in any nilpotent ideal (nor even in a nil ideal). For example, in the general matrix ring over a field there are nilpotent elements; in particular, the nilpotent matrices mentioned above, in which the only non-zero elements stand above the main diagonal, but since the ring is simple, it has no non-zero nilpotent ideals.
In a finite-dimensional Lie algebra $G$ there is maximal nilpotent ideal, which consists of the elements $x\in G$ for which the endomorphism $y\to[x,y]$ for $y\in G$ is nilpotent.