Home > Product of rings

(a_i) + (b_i) = (a_i + b_i)

(a_i) · (b_i) = (a_i · b_i)

The product of finitely many rings R₁,...,R_k is also written as R₁ × R₂ × ... × R_k.

1 Examples

The most important example is the ring Z/nZ of integers modulo n. If n is written as a product of prime powers (see fundamental theorem of arithmetic):

where the p_i are distinct primes, then Z/nZ is naturally isomorphic to the product ring

This follows from the Chinese remainder theorem.

2 Properties

If R = Π_{i in I} R_i is a product of rings, then for every i in I we have a surjective ring homomorphism p_i : R -> R_i which projects the product on the i-th coordinate. The product R, together with the projections p_i, has the following universal property:

if S is any ring and f_i : S -> R_i is a ring homomorphism for every i in I, then there exists precisely one ring homomorphism f : S -> R such that p_i o f = f_i for every i in I.

This shows that the product of rings is an instance of products in the sense of category theory.

If A is a (left, right, two-sided) ideal in R, then there exist (left, right, two-sided) ideals A_i in R_i such that A = Π_{i in I} A_i. Conversely, every such product of ideals is an ideal in R. A is a prime idealIn mathematics, a prime ideal is a subset of a ring which shares many important properties of a prime number in the ring of integers. Prime ideals have a simpler description for commutative rings, so we consider this case separately below. This article on in R if and only if all but one of the A_i are equal to R_i and the remaining A_i is a prime ideal in R_i.

An element x in R is a unit if and only if all of its components are units, i.e. if and only if p_i(x) is a unit in R_i for every i in I. The group of units of R is the productIn mathematics, given a group G and two subgroups H and K of G one can define the product of H and K denoted by HK as the set of all elements of the form hk for all h in H and k in K''. In general HK is not a subgroup hkh'k' is not of the form hk ; it is of the groups of units of R_i.

A product of more than one non-zero rings always has zero divisors: if x is an element of the product all of whose coordinates are zero except p_i(x), and y is an element of the product with all coordinates zero except p_j(y) (with i ≠ j), then xy = 0 in the product ring.