# Subring

Short description: Subset of a ring that forms a ring itself

In mathematics, a subring of R is a subset of a ring that is itself a ring when binary operations of addition and multiplication on R are restricted to the subset, and which shares the same multiplicative identity as R. (Note that a subset of a ring R need not be a ring.) For those who define rings without requiring the existence of a multiplicative identity, a subring of R is just a subset of R that is a ring for the operations of R (this does imply it contains the additive identity of R). The latter gives a strictly weaker condition, even for rings that do have a multiplicative identity, so that for instance all ideals become subrings (and they may have a multiplicative identity that differs from the one of R). With definition requiring a multiplicative identity (which is used in this article), the only ideal of R that is a subring of R is R itself.

## Definition

A subring of a ring (R, +, ∗, 0, 1) is a subset S of R that preserves the structure of the ring, i.e. a ring (S, +, ∗, 0, 1) with SR. Equivalently, it is both a subgroup of (R, +, 0) and a submonoid of (R, ∗, 1).

## Examples

The ring $\displaystyle{ \mathbb{Z} }$ and its quotients $\displaystyle{ \mathbb{Z}/n\mathbb{Z} }$ have no subrings (with multiplicative identity) other than the full ring.[1]:228

Every ring has a unique smallest subring, isomorphic to some ring $\displaystyle{ \mathbb{Z}/n\mathbb{Z} }$ with n a nonnegative integer (see Characteristic). The integers $\displaystyle{ \mathbb{Z} }$ correspond to n = 0 in this statement, since $\displaystyle{ \mathbb{Z} }$ is isomorphic to $\displaystyle{ \mathbb{Z}/0\mathbb{Z} }$.[2]:89-90

## Subring test

The subring test is a theorem that states that for any ring R, a subset S of R is a subring if and only if it contains the multiplicative identity of R, and is closed under multiplication and subtraction.[1]:228

As an example, the ring Z of integers is a subring of the field of real numbers and also a subring of the ring of polynomials Z[X].

## Center

The center of a ring is the set of the elements of the ring that commute with every other element of the ring. That is, x belongs to the center of the ring R if $\displaystyle{ xy=yx }$ for every $\displaystyle{ y\in x. }$

The center of a ring R is a subring of R, and R is an associative algebra over its center.

## Prime subring

The intersection of all subrings of a ring R is a subring that may be called the prime subring of R by analogy with prime fields.

The prime subring of a ring R is a subring of the center of R, which is isomorphic either to the ring $\displaystyle{ \Z }$ of the integers or to the ring of the integers modulo n, where n is the smallest positive integer such that the sum of n copies of 1 equals 0.

## Ring extensions

If S is a subring of a ring R, then equivalently R is said to be a ring extension of S, written as R/S in similar notation to that for field extensions.

## Subring generated by a set

Let R be a ring. Any intersection of subrings of R is again a subring of R. Therefore, if X is any subset of R, the intersection of all subrings of R containing X is a subring S of R. S is the smallest subring of R containing X. ("Smallest" means that if T is any other subring of R containing X, then S is contained in T.) S is said to be the subring of R generated by X. If S = R, we may say that the ring R is generated by X.