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单词 GroupOfUnits
释义

group of units


Theorem.

The set E of units of a ring R forms a group with respect to ring multiplication.

Proof.  If u and v are two units, then there are the elements r and s of R such that  ru=ur=1  and  sv=vs=1.  Then we get that (sr)(uv)=s(r(uv))=s((ru)v)=s(1v)=sv=1,  similarly  (uv)(sr)=1.  Thus also uv is a unit, which means that E is closed under multiplication.  Because  1E  and along with u also its inverseMathworldPlanetmathPlanetmathPlanetmathPlanetmath r belongs to E, the set E is a group.

Corollary.  In a commutative ring, a ring product is a unit iff all are units.

The group E of the units of the ring R is called the group of units of the ring.  If R is a field, E is said to be the multiplicative groupMathworldPlanetmath of the field.

Examples

  1. 1.

    When  R=, then  E={1,-1}.

  2. 2.

    When  R=[i],  the ring of Gaussian integersMathworldPlanetmath, then E={1,i,-1,-i}.

  3. 3.

    When  R=[3], then (http://planetmath.org/UnitsOfQuadraticFields) E={±(2+3)nn}.

  4. 4.

    When  R=K[X]  where K is a field, then  E=K{0}.

  5. 5.

    When R={0+, 1+,,m-1+}  is the residue class ring modulo m, then  E consists of the prime classes modulo m, i.e. the residue classesMathworldPlanetmath l+ satisfying  gcd(l,m)=1.

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