In ring theory, a ring R is called a reduced ring if it has no non-zero nilpotent elements. Equivalently, a ring is reduced if it has no non-zero elements with square zero, that is, x2 = 0 implies x = 0. A commutative algebra over a commutative ring is called a reduced algebra if its underlying ring is reduced.
The nilpotent elements of a commutative ring R form an ideal of R, called the nilradical of R; therefore a commutative ring is reduced if and only if its nilradical is zero. Moreover, a commutative ring is reduced if and only if the only element contained in all prime ideals is zero.
Over a Noetherian ring R, we say a finitely generated module M has locally constant rank if is a locally constant (or equivalently continuous) function on Spec R. Then R is reduced if and only if every finitely generated module of locally constant rank is projective.
Examples and non-examples
- Subrings, products, and localizations of reduced rings are again reduced rings.
- The ring of integers Z is a reduced ring. Every field and every polynomial ring over a field (in arbitrarily many variables) is a reduced ring.
- More generally, every integral domain is a reduced ring since a nilpotent element is a fortiori a zero divisor. On the other hand, not every reduced ring is an integral domain. For example, the ring Z[x, y]/(xy) contains x + (xy) and y + (xy) as zero divisors, but no non-zero nilpotent elements. As another example, the ring Z×Z contains (1,0) and (0,1) as zero divisors, but contains no non-zero nilpotent elements.
- The ring Z/6Z is reduced, however Z/4Z is not reduced: The class 2 + 4Z is nilpotent. In general, Z/nZ is reduced if and only if n = 0 or n is a square-free integer.
- If R is a commutative ring and N is the nilradical of R, then the quotient ring R/N is reduced.
- A commutative ring R of characteristic p for some prime number p is reduced if and only if its Frobenius endomorphism is injective. (cf. perfect field.)
- Proof: let be all the (possibly zero) minimal prime ideals.
- Let x be in D. Then xy = 0 for some nonzero y. Since R is reduced, (0) is the intersection of all and thus y is not in some . Since xy is in all ; in particular, in , x is in .
- (stolen from Kaplansky, commutative rings, Theorem 84). We drop the subscript i. Let . S is multiplicatively closed and so we can consider the localization . Let be the pre-image of a maximal ideal. Then is contained in both D and and by minimality . (This direction is immediate if R is Noetherian by the theory of associated primes.)
- Eisenbud, Exercise 20.13.