Reduced ring

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.

A quotient ring R/I is reduced if and only if I is a radical ideal.

denote nilradical of a commutative ring

and it is left adjoint to the inclusion functor

is induced from the universal property of quotient rings.

Let D be the set of all zero-divisors in a reduced ring R. Then D is the union of all minimal prime ideals.

[1] 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.

[2] Reduced rings play an elementary role in algebraic geometry, where this concept is generalized to the notion of a reduced scheme.