group of units
Theorem.
The set of units of a ring forms a group with respect to ring multiplication.
Proof. If and are two units, then there are the elements and of such that and . Then we get that , similarly . Thus also is a unit, which means that is closed under multiplication. Because and along with also its inverse belongs to , the set is a group.
Corollary. In a commutative ring, a ring product is a unit iff all are units.
The group of the units of the ring is called the group of units of the ring. If is a field, is said to be the multiplicative group of the field.
Examples
- 1.
When , then .
- 2.
When , the ring of Gaussian integers
, then .
- 3.
When , then (http://planetmath.org/UnitsOfQuadraticFields) .
- 4.
When where is a field, then .
- 5.
When is the residue class ring modulo , then consists of the prime classes modulo , i.e. the residue classes
satisfying .