# Nilpotent ideal

In mathematics, more specifically ring theory, an ideal, *I*, of a ring is said to be a **nilpotent ideal**, if there exists a natural number *k* such that *I*^{k} = 0.[1] By *I*^{k}, it is meant the additive subgroup generated by the set of all products of *k* elements in *I*.[1] Therefore, *I* is nilpotent if and only if there is a natural number *k* such that the product of any *k* elements of *I* is 0.

The notion of a nilpotent ideal is much stronger than that of a nil ideal in many classes of rings. There are, however, instances when the two notions coincide—this is exemplified by Levitzky's theorem.[2][3] The notion of a nilpotent ideal, although interesting in the case of commutative rings, is most interesting in the case of noncommutative rings.

## Relation to nil ideals

The notion of a nil ideal has a deep connection with that of a nilpotent ideal, and in some classes of rings, the two notions coincide. If an ideal is nilpotent, it is of course nil, but a nil ideal need not be nilpotent for more than one reason. The first is that there need not be a global upper bound on the exponent required to annihilate various elements of the nil ideal, and secondly, each element being nilpotent does not force products of distinct elements to vanish.[1]

In a right artinian ring, any nil ideal is nilpotent.[4] This is proven by observing that any nil ideal is contained in the Jacobson radical of the ring, and since the Jacobson radical is a nilpotent ideal (due to the artinian hypothesis), the result follows. In fact, this can be generalized to right noetherian rings; this result is known as Levitzky's theorem.[3]

## Notes

- Isaacs 1993, p. 194.
- Isaacs, Theorem 14.38, p. 210
- Herstein 1968, Theorem 1.4.5, p. 37.
- Isaacs, Corollary 14.3, p. 195

## References

- I.N. Herstein (1968).
*Noncommutative rings*(1st ed.). The Mathematical Association of America. ISBN 0-88385-015-X. - I. Martin Isaacs (1993).
*Algebra, a graduate course*(1st ed.). Brooks/Cole Publishing Company. ISBN 0-534-19002-2.