In the branch of abstract algebra known as ring theory, a **unit** of a ring is any element that has a multiplicative inverse in : an element such that

- ,

where 1 is the multiplicative identity.^{[1]}^{[2]} The set of units U(*R*) of a ring forms a group under multiplication.

Less commonly, the term *unit* is also used to refer to the element 1 of the ring, in expressions like *ring with a unit* or *unit ring*, and also e.g. *'unit' matrix*. For this reason, some authors call 1 "unity" or "identity", and say that R is a "ring with unity" or a "ring with identity" rather than a "ring with a unit".

## Examples

The multiplicative identity 1 and its additive inverse −1 are always units. More generally, any root of unity in a ring *R* is a unit: if *r ^{n}* = 1, then

*r*

^{n − 1}is a multiplicative inverse of r. In a nonzero ring, the element 0 is not a unit, so U(

*R*) is not closed under addition. A ring R in which every nonzero element is a unit (that is, U(

*R*) =

*R*−{0}) is called a division ring (or a skew-field). A commutative division ring is called a field. For example, the unit group of the field of real numbers

**R**is

**R**− {0}.

### Integers

In the ring of integers **Z**, the only units are 1 and −1.

The ring of integers in a number field may have more units in general. For example, in the ring **Z**[1 + √5/ 2] that arises by adjoining the quadratic integer 1 + √5/ 2 to **Z**, one has

- (√5 + 2)(√5 − 2) = 1

in the ring, so √5 + 2 is a unit. (In fact, the unit group of this ring is infinite.^{[citation needed]})

In fact, Dirichlet's unit theorem describes the structure of U(*R*) precisely: it is isomorphic to a group of the form

where is the (finite, cyclic) group of roots of unity in R and n, the rank of the unit group is

where are the numbers of real embeddings and the number of pairs of complex embeddings of F, respectively.

This recovers the above example: the unit group of (the ring of integers of) a real quadratic field is infinite of rank 1, since .

In the ring **Z**/*n***Z** of integers modulo n, the units are the congruence classes (mod *n*) represented by integers coprime to n. They constitute the multiplicative group of integers modulo n.

### Polynomials and power series

For a commutative ring R, the units of the polynomial ring *R*[*x*] are precisely those polynomials

such that is a unit in R, and the remaining coefficients are nilpotent elements, i.e., satisfy for some *N*.^{[3]}
In particular, if *R* is a domain (has no zero divisors), then the units of *R*[*x*] agree with the ones of R.
The units of the power series ring are precisely those power series

such that is a unit in R.^{[4]}

### Matrix rings

The unit group of the ring M_{n}(*R*) of *n* × *n* matrices over a ring R is the group GL_{n}(*R*) of invertible matrices. For a commutative ring R, an element A of M_{n}(*R*) is invertible if and only if the determinant of A is invertible in R. In that case, *A*^{−1} is explicitly given by Cramer's rule.

### In general

For elements x and y in a ring R, if is invertible, then is invertible with the inverse .^{[5]} The formula for the inverse can be guessed, but not proved, by the following calculation in a ring of noncommutative power series:

See Hua's identity for similar results.

## Group of units

The units of a ring R form a group U(*R*) under multiplication, the *group of units* of R.

Other common notations for U(*R*) are *R*^{∗}, *R*^{×}, and E(*R*) (from the German term *Einheit*).

A commutative ring is a local ring if *R* − U(*R*) is a maximal ideal.

As it turns out, if *R* − U(*R*) is an ideal, then it is necessarily a maximal ideal and *R* is local since a maximal ideal is disjoint from U(*R*).

If R is a finite field, then U(*R*) is a cyclic group of order .

The formulation of the group of units defines a functor U from the category of rings to the category of groups:

every ring homomorphism *f* : *R* → *S* induces a group homomorphism U(*f*) : U(*R*) → U(*S*), since f maps units to units.

This functor has a left adjoint which is the integral group ring construction.^{[6]}

The group scheme is isomorphic to the multiplicative group scheme over any base, so for any commutative ring R, the groups and are canonically isomorphic to . Note that the functor (that is, ) is representable in the sense: for commutative rings *R* (this for instance follows from the aforementioned adjoint relation with the group ring construction). Explicitly this means that there is a natural bijection between the set of the ring homomorphisms and the set of unit elements of *R* (in contrast, represents the additive group , the forgetful functor from the category of commutative rings to the category of abelian groups).

## Associatedness

Suppose that R is commutative. Elements r and s of R are called *associate* if there exists a unit u in R such that *r* = *us*; then write *r* ∼ *s*. In any ring, pairs of additive inverse elements^{[a]} *x* and −*x* are associate. For example, 6 and −6 are associate in **Z**. In general, ~ is an equivalence relation on R.

Associatedness can also be described in terms of the action of U(*R*) on R via multiplication: Two elements of R are associate if they are in the same U(*R*)-orbit.

In an integral domain, the set of associates of a given nonzero element has the same cardinality as U(*R*).

The equivalence relation ~ can be viewed as any one of Green's semigroup relations specialized to the multiplicative semigroup of a commutative ring R.

## See also

## Notes

**^**x and −*x*are not necessarily distinct. For example, in the ring of integers modulo 6, one has 3 = −3 even though 1 ≠ −1.

### Citations

**^**Dummit & Foote 2004.**^**Lang 2002.**^**Watkins (2007, Theorem 11.1)**^**Watkins (2007, Theorem 12.1)**^**Jacobson 2009, § 2.2. Exercise 4.**^**Exercise 10 in § 2.2. of Cohn, Paul M. (2003).*Further algebra and applications*(Revised ed. of Algebra, 2nd ed.). London: Springer-Verlag. ISBN 1-85233-667-6. Zbl 1006.00001.

## Sources

- Dummit, David S.; Foote, Richard M. (2004).
*Abstract Algebra*(3rd ed.). John Wiley & Sons. ISBN 0-471-43334-9. - Jacobson, Nathan (2009).
*Basic Algebra 1*(2nd ed.). Dover. ISBN 978-0-486-47189-1. - Lang, Serge (2002).
*Algebra*. Graduate Texts in Mathematics. Springer. ISBN 0-387-95385-X. - Watkins, John J. (2007),
*Topics in commutative ring theory*, Princeton University Press, ISBN 978-0-691-12748-4, MR 2330411