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56 Rings
56.1 Generating Rings
56.1-1 IsRing
56.1-2 Ring
56.1-3 DefaultRing
56.1-4 RingByGenerators
56.1-5 DefaultRingByGenerators
56.1-6 GeneratorsOfRing
56.1-7 Subring
56.1-8 ClosureRing
56.1-9 Quotient
56.2 Ideals of Rings
56.2-1 TwoSidedIdeal
56.2-2 TwoSidedIdealNC
56.2-3 IsTwoSidedIdeal
56.2-4 TwoSidedIdealByGenerators
56.2-5 LeftIdealByGenerators
56.2-6 RightIdealByGenerators
56.2-7 GeneratorsOfTwoSidedIdeal
56.2-8 GeneratorsOfLeftIdeal
56.2-9 GeneratorsOfRightIdeal
56.2-10 LeftActingRingOfIdeal
56.2-11 AsLeftIdeal
56.3 Rings With One
56.3-1 IsRingWithOne
56.3-2 RingWithOne
56.3-3 RingWithOneByGenerators
56.3-4 GeneratorsOfRingWithOne
56.3-5 SubringWithOne
56.4 Properties of Rings
56.4-1 IsIntegralRing
56.4-2 IsUniqueFactorizationRing
56.4-3 IsLDistributive
56.4-4 IsRDistributive
56.4-5 IsDistributive
56.4-6 IsAnticommutative
56.4-7 IsZeroSquaredRing
56.4-8 IsJacobianRing
56.5 Units and Factorizations
56.5-1 IsUnit
56.5-2 Units
56.5-3 IsAssociated
56.5-4 Associates
56.5-5 StandardAssociate
56.5-6 StandardAssociateUnit
56.5-7 IsIrreducibleRingElement
56.5-8 IsPrime
56.5-9 Factors
56.5-10 PadicValuation
56.6 Euclidean Rings
56.6-1 IsEuclideanRing
56.6-2 EuclideanDegree
56.6-3 EuclideanQuotient
56.6-4 EuclideanRemainder
56.6-5 QuotientRemainder
56.7 Gcd and Lcm
56.7-1 Gcd
56.7-2 GcdOp
56.7-3 GcdRepresentation
56.7-4 GcdRepresentationOp
56.7-5 ShowGcd
56.7-6 Lcm
56.7-7 LcmOp
56.7-8 QuotientMod
56.7-9 PowerMod
56.7-10 InterpolatedPolynomial
56.8 Homomorphisms of Rings
56.8-1 RingGeneralMappingByImages
56.8-2 RingHomomorphismByImages
56.8-3 RingHomomorphismByImagesNC
56.8-4 NaturalHomomorphismByIdeal
56.9 Small Rings
56.9-1 SmallRing
56.9-2 NumberSmallRings
56.9-3 Subrings
56.9-4 Ideals
56.9-5 DirectSum
56.9-6 RingByStructureConstants

### 56 Rings

This chapter deals with domains that are additive groups (see `IsAdditiveGroup` (55.1-6) closed under multiplication `*`. Such a domain, if `*` and `+` are distributive, is called a ring in GAP. Each division ring, field (see 58), or algebra (see 62) is a ring. Important examples of rings are the integers (see 14) and matrix rings.

In the case of a ring-with-one, additional multiplicative structure is present, see `IsRingWithOne` (56.3-1). There is a little support in GAP for rings that have no additional structure: it is possible to perform some computations for small finite rings; infinite rings are handled by GAP in an acceptable way in the case that they are algebras.

Also, the SONATA package provides support for near-rings, and a related functionality for multiplicative semigroups of near-rings is available in the Smallsemi package.

Several functions for ring elements, such as `IsPrime` (56.5-8) and `Factors` (56.5-9), are defined only relative to a ring R, which can be entered as an optional argument; if R is omitted then a default ring is formed from the ring elements given as arguments, see `DefaultRing` (56.1-3).

#### 56.1 Generating Rings

##### 56.1-1 IsRing

`‣ IsRing`( R ) | ( filter )

A ring in GAP is an additive group (see `IsAdditiveGroup` (55.1-6)) that is also a magma (see `IsMagma` (35.1-1)), such that addition `+` and multiplication `*` are distributive, see `IsDistributive` (56.4-5).

The multiplication need not be associative (see `IsAssociative` (35.4-7)). For example, a Lie algebra (see 64) is regarded as a ring in GAP.

##### 56.1-2 Ring

`‣ Ring`( r, s, ... ) | ( function )

`‣ Ring`( coll ) | ( function )

In the first form `Ring` returns the smallest ring that contains all the elements r, s, ... In the second form `Ring` returns the smallest ring that contains all the elements in the collection coll. If any element is not an element of a ring or if the elements lie in no common ring an error is raised.

`Ring` differs from `DefaultRing` (56.1-3) in that it returns the smallest ring in which the elements lie, while `DefaultRing` (56.1-3) may return a larger ring if that makes sense.

    gap> Ring( 2, E(4) );
    <ring with 2 generators>

##### 56.1-3 DefaultRing

`‣ DefaultRing`( r, s, ... ) | ( function )

`‣ DefaultRing`( coll ) | ( function )

In the first form `DefaultRing` returns a ring that contains all the elements r, s, ... etc. In the second form `DefaultRing` returns a ring that contains all the elements in the collection coll. If any element is not an element of a ring or if the elements lie in no common ring an error is raised.

The ring returned by `DefaultRing` need not be the smallest ring in which the elements lie. For example for elements from cyclotomic fields, `DefaultRing` may return the ring of integers of the smallest cyclotomic field in which the elements lie, which need not be the smallest ring overall, because the elements may in fact lie in a smaller number field which is itself not a cyclotomic field.

(For the exact definition of the default ring of a certain type of elements, look at the corresponding method installation.)

`DefaultRing` is used by ring functions such as `Quotient` (56.1-9), `IsPrime` (56.5-8), `Factors` (56.5-9), or `Gcd` (56.7-1) if no explicit ring is given.

`Ring` (56.1-2) differs from `DefaultRing` in that it returns the smallest ring in which the elements lie, while `DefaultRing` may return a larger ring if that makes sense.

    gap> DefaultRing( 2, E(4) );
    GaussianIntegers

##### 56.1-4 RingByGenerators

`‣ RingByGenerators`( C ) | ( operation )

`RingByGenerators` returns the ring generated by the elements in the collection C, i. e., the closure of C under addition, multiplication, and taking additive inverses.

    gap> RingByGenerators([ 2, E(4) ]);
    <ring with 2 generators>

##### 56.1-5 DefaultRingByGenerators

`‣ DefaultRingByGenerators`( coll ) | ( operation )

For a collection coll, returns a default ring in which coll is contained.

    gap> DefaultRingByGenerators([ 2, E(4) ]);
    GaussianIntegers

##### 56.1-6 GeneratorsOfRing

`‣ GeneratorsOfRing`( R ) | ( attribute )

`GeneratorsOfRing` returns a list of elements such that the ring R is the closure of these elements under addition, multiplication, and taking additive inverses.

    gap> R:=Ring( 2, 1/2 );
    <ring with 2 generators>
    gap> GeneratorsOfRing( R );
    [ 2, 1/2 ]

##### 56.1-7 Subring

`‣ Subring`( R, gens ) | ( function )

`‣ SubringNC`( R, gens ) | ( function )

returns the ring with parent R generated by the elements in gens. When the second form, `SubringNC` is used, it is not checked whether all elements in gens lie in R.

    gap> R:= Integers;
    Integers
    gap> S:= Subring( R, [ 4, 6 ] );
    <ring with 1 generator>
    gap> Parent( S );
    Integers

##### 56.1-8 ClosureRing

`‣ ClosureRing`( R, r ) | ( operation )

`‣ ClosureRing`( R, S ) | ( operation )

For a ring R and either an element r of its elements family or a ring S, `ClosureRing` returns the ring generated by both arguments.

    gap> ClosureRing( Integers, E(4) );
    <ring-with-one, with 2 generators>

##### 56.1-9 Quotient

`‣ Quotient`( [R, ]r, s ) | ( operation )

`Quotient` returns a (right) quotient of the two ring elements r and s in the ring R, if given, and otherwise in their default ring (see `DefaultRing` (56.1-3)). More specifically, it returns a ring element q such that r = q * s holds, or `fail` if no such elements exists in the respective ring.

The result may not be unique if the ring contains zero divisors.

(To perform the division in the quotient field of a ring, use the quotient operator `/`.)

    gap> Quotient( 2, 3 );
    fail
    gap> Quotient( 6, 3 );
    2

#### 56.2 Ideals of Rings

A left ideal in a ring R is a subring of R that is closed under multiplication with elements of R from the left.

A right ideal in a ring R is a subring of R that is closed under multiplication with elements of R from the right.

A two-sided ideal or simply ideal in a ring R is both a left ideal and a right ideal in R.

So being a (left/right/two-sided) ideal is not a property of a domain but refers to the acting ring(s). Hence we must ask, e. g., `IsIdeal(` R, I`)` if we want to know whether the ring I is an ideal in the ring R. The property `IsTwoSidedIdealInParent` (56.2-3) can be used to store whether a ring is an ideal in its parent.

(Whenever the term `"Ideal"` occurs in an identifier without a specifying prefix `"Left"` or `"Right"`, this means the same as `"TwoSidedIdeal"`. Conversely, any occurrence of `"TwoSidedIdeal"` can be substituted by `"Ideal"`.)

For any of the above kinds of ideals, there is a notion of generators, namely `GeneratorsOfLeftIdeal` (56.2-8), `GeneratorsOfRightIdeal` (56.2-9), and `GeneratorsOfTwoSidedIdeal` (56.2-7). The acting rings can be accessed as `LeftActingRingOfIdeal` (56.2-10) and `RightActingRingOfIdeal` (56.2-10), respectively. Note that ideals are detected from known values of these attributes, especially it is assumed that whenever a domain has both a left and a right acting ring then these two are equal.

Note that we cannot use `LeftActingDomain` (57.1-11) and `RightActingDomain` here, since ideals in algebras are themselves vector spaces, and such a space can of course also be a module for an action from the right. In order to make the usual vector space functionality automatically available for ideals, we have to distinguish the left and right module structure from the additional closure properties of the ideal.

Further note that the attributes denoting ideal generators and acting ring are used to create ideals if this is explicitly wanted, but the ideal relation in the sense of `IsTwoSidedIdeal` (56.2-3) is of course independent of the presence of the attribute values.

Ideals are constructed with `LeftIdeal` (56.2-1), `RightIdeal` (56.2-1), `TwoSidedIdeal` (56.2-1). Principal ideals of the form x * R, R * x, R * x * R can also be constructed with a simple multiplication.

Currently many methods for dealing with ideals need linear algebra to work, so they are mainly applicable to ideals in algebras.

##### 56.2-1 TwoSidedIdeal

`‣ TwoSidedIdeal`( R, gens[, "basis"] ) | ( function )

`‣ Ideal`( R, gens[, "basis"] ) | ( function )

`‣ LeftIdeal`( R, gens[, "basis"] ) | ( function )

`‣ RightIdeal`( R, gens[, "basis"] ) | ( function )

Let R be a ring, and gens a list of collection of elements in R. `TwoSidedIdeal`, `LeftIdeal`, and `RightIdeal` return the two-sided, left, or right ideal, respectively, I in R that is generated by gens. The ring R can be accessed as `LeftActingRingOfIdeal` (56.2-10) or `RightActingRingOfIdeal` (56.2-10) (or both) of I.

If R is a left F-module then also I is a left F-module, in particular the `LeftActingDomain` (57.1-11) values of R and I are equal.

If the optional argument `"basis"` is given then gens are assumed to be a list of basis vectors of I viewed as a free F-module. (This is mainly applicable to ideals in algebras.) In this case, it is not checked whether gens really is linearly independent and whether gens is a subset of R.

`Ideal` is simply a synonym of `TwoSidedIdeal`.

    gap> R:= Integers;;
    gap> I:= Ideal( R, [ 2 ] );
    <two-sided ideal in Integers, (1 generator)>

##### 56.2-2 TwoSidedIdealNC

`‣ TwoSidedIdealNC`( R, gens[, "basis"] ) | ( function )

`‣ IdealNC`( R, gens[, "basis"] ) | ( function )

`‣ LeftIdealNC`( R, gens[, "basis"] ) | ( function )

`‣ RightIdealNC`( R, gens[, "basis"] ) | ( function )

The effects of `TwoSidedIdealNC`, `LeftIdealNC`, and `RightIdealNC` are the same as `TwoSidedIdeal` (56.2-1), `LeftIdeal` (56.2-1), and `RightIdeal` (56.2-1), respectively, but they do not check whether all entries of gens lie in R.

##### 56.2-3 IsTwoSidedIdeal

`‣ IsTwoSidedIdeal`( R, I ) | ( operation )

`‣ IsLeftIdeal`( R, I ) | ( operation )

`‣ IsRightIdeal`( R, I ) | ( operation )

`‣ IsTwoSidedIdealInParent`( I ) | ( property )

`‣ IsLeftIdealInParent`( I ) | ( property )

`‣ IsRightIdealInParent`( I ) | ( property )

The properties `IsTwoSidedIdealInParent` etc., are attributes of the ideal, and once known they are stored in the ideal.

    gap> A:= FullMatrixAlgebra( Rationals, 3 );
    ( Rationals^[ 3, 3 ] )
    gap> I:= Ideal( A, [ Random( A ) ] );
    <two-sided ideal in ( Rationals^[ 3, 3 ] ), (1 generator)>
    gap> IsTwoSidedIdeal( A, I );
    true

##### 56.2-4 TwoSidedIdealByGenerators

`‣ TwoSidedIdealByGenerators`( R, gens ) | ( operation )

`‣ IdealByGenerators`( R, gens ) | ( operation )

`TwoSidedIdealByGenerators` returns the ring that is generated by the elements of the collection gens under addition, multiplication, and multiplication with elements of the ring R from the left and from the right.

R can be accessed by `LeftActingRingOfIdeal` (56.2-10) or `RightActingRingOfIdeal` (56.2-10), gens can be accessed by `GeneratorsOfTwoSidedIdeal` (56.2-7).

##### 56.2-5 LeftIdealByGenerators

`‣ LeftIdealByGenerators`( R, gens ) | ( operation )

`LeftIdealByGenerators` returns the ring that is generated by the elements of the collection gens under addition, multiplication, and multiplication with elements of the ring R from the left.

R can be accessed by `LeftActingRingOfIdeal` (56.2-10), gens can be accessed by `GeneratorsOfLeftIdeal` (56.2-8).

##### 56.2-6 RightIdealByGenerators

`‣ RightIdealByGenerators`( R, gens ) | ( operation )

`RightIdealByGenerators` returns the ring that is generated by the elements of the collection gens under addition, multiplication, and multiplication with elements of the ring R from the right.

R can be accessed by `RightActingRingOfIdeal` (56.2-10), gens can be accessed by `GeneratorsOfRightIdeal` (56.2-9).

##### 56.2-7 GeneratorsOfTwoSidedIdeal

`‣ GeneratorsOfTwoSidedIdeal`( I ) | ( attribute )

`‣ GeneratorsOfIdeal`( I ) | ( attribute )

is a list of generators for the ideal I, with respect to the action of the rings that are stored as the values of `LeftActingRingOfIdeal` (56.2-10) and `RightActingRingOfIdeal` (56.2-10), from the left and from the right, respectively.

    gap> A:= FullMatrixAlgebra( Rationals, 3 );;
    gap> I:= Ideal( A, [ One( A ) ] );;
    gap> GeneratorsOfIdeal( I );
    [ [ [ 1, 0, 0 ], [ 0, 1, 0 ], [ 0, 0, 1 ] ] ]

##### 56.2-8 GeneratorsOfLeftIdeal

`‣ GeneratorsOfLeftIdeal`( I ) | ( attribute )

is a list of generators for the left ideal I, with respect to the action from the left of the ring that is stored as the value of `LeftActingRingOfIdeal` (56.2-10).

##### 56.2-9 GeneratorsOfRightIdeal

`‣ GeneratorsOfRightIdeal`( I ) | ( attribute )

is a list of generators for the right ideal I, with respect to the action from the right of the ring that is stored as the value of `RightActingRingOfIdeal` (56.2-10).

##### 56.2-10 LeftActingRingOfIdeal

`‣ LeftActingRingOfIdeal`( I ) | ( attribute )

`‣ RightActingRingOfIdeal`( I ) | ( attribute )

returns the left (resp. right) acting ring of an ideal I.

##### 56.2-11 AsLeftIdeal

`‣ AsLeftIdeal`( R, S ) | ( operation )

`‣ AsRightIdeal`( R, S ) | ( operation )

`‣ AsTwoSidedIdeal`( R, S ) | ( operation )

Let S be a subring of the ring R.

If S is a left ideal in R then `AsLeftIdeal` returns this left ideal, otherwise `fail` is returned.

If S is a right ideal in R then `AsRightIdeal` returns this right ideal, otherwise `fail` is returned.

If S is a two-sided ideal in R then `AsTwoSidedIdeal` returns this two-sided ideal, otherwise `fail` is returned.

    gap> A:= FullMatrixAlgebra( Rationals, 3 );;
    gap> B:= DirectSumOfAlgebras( A, A );
    <algebra over Rationals, with 6 generators>
    gap> C:= Subalgebra( B, Basis( B ){[1..9]} );
    <algebra over Rationals, with 9 generators>
    gap> I:= AsTwoSidedIdeal( B, C );
    <two-sided ideal in <algebra of dimension 18 over Rationals>,
      (9 generators)>

#### 56.3 Rings With One

##### 56.3-1 IsRingWithOne

`‣ IsRingWithOne`( R ) | ( filter )

A ring-with-one in GAP is a ring (see `IsRing` (56.1-1)) that is also a magma-with-one (see `IsMagmaWithOne` (35.1-2)).

Note that the identity and the zero of a ring-with-one need not be distinct. This means that a ring that consists only of its zero element can be regarded as a ring-with-one.

This is especially useful in the case of finitely presented rings, in the sense that each factor of a ring-with-one is again a ring-with-one.

##### 56.3-2 RingWithOne

`‣ RingWithOne`( r, s, ... ) | ( function )

`‣ RingWithOne`( coll ) | ( function )

In the first form `RingWithOne` returns the smallest ring with one that contains all the elements r, s, ... In the second form `RingWithOne` returns the smallest ring with one that contains all the elements in the collection C. If any element is not an element of a ring or if the elements lie in no common ring an error is raised.

    gap> RingWithOne( [ 4, 6 ] );
    Integers

##### 56.3-3 RingWithOneByGenerators

`‣ RingWithOneByGenerators`( coll ) | ( operation )

`RingWithOneByGenerators` returns the ring-with-one generated by the elements in the collection coll, i. e., the closure of coll under addition, multiplication, taking additive inverses, and taking the identity of an element.

##### 56.3-4 GeneratorsOfRingWithOne

`‣ GeneratorsOfRingWithOne`( R ) | ( attribute )

`GeneratorsOfRingWithOne` returns a list of elements such that the ring R is the closure of these elements under addition, multiplication, taking additive inverses, and taking the identity element `One( R )`.

R itself need not be known to be a ring-with-one.

    gap> R:= RingWithOne( [ 4, 6 ] );
    Integers
    gap> GeneratorsOfRingWithOne( R );
    [ 1 ]

##### 56.3-5 SubringWithOne

`‣ SubringWithOne`( R, gens ) | ( function )

`‣ SubringWithOneNC`( R, gens ) | ( function )

returns the ring with one with parent R generated by the elements in gens. When the second form, `SubringWithOneNC` is used, it is not checked whether all elements in gens lie in R.

    gap> R:= SubringWithOne( Integers, [ 4, 6 ] );
    Integers
    gap> Parent( R );
    Integers

#### 56.4 Properties of Rings

##### 56.4-1 IsIntegralRing

`‣ IsIntegralRing`( R ) | ( property )

A ring-with-one R is integral if it is commutative, contains no nontrivial zero divisors, and if its identity is distinct from its zero.

    gap> IsIntegralRing( Integers );
    true

##### 56.4-2 IsUniqueFactorizationRing

`‣ IsUniqueFactorizationRing`( R ) | ( category )

A ring R is called a unique factorization ring if every nonzero element has a unique factorization into irreducible elements, i.e., a unique representation as product of irreducibles (see `IsIrreducibleRingElement` (56.5-7)). Unique in this context means unique up to permutations of the factors and up to multiplication of the factors by units (see `Units` (56.5-2)).

(Note that we cannot install a subset maintained method for this filter since the factorization of an element needs not exist in a subring. As an example, consider the subring 4 ℕ + 1 of the ring 4 ℤ + 1; in the subring, the element 3 ⋅ 3 ⋅ 11 ⋅ 7 has the two factorizations 33 ⋅ 21 = 9 ⋅ 77, but in the large ring there is the unique factorization (-3) ⋅ (-3) ⋅ (-11) ⋅ (-7), and it is easy to see that every element in 4 ℤ + 1 has a unique factorization.)

    gap> IsUniqueFactorizationRing( PolynomialRing( Rationals, 1 ) );
    true

##### 56.4-3 IsLDistributive

`‣ IsLDistributive`( C ) | ( property )

is `true` if the relation a * ( b + c ) = ( a * b ) + ( a * c ) holds for all elements a, b, c in the collection C, and `false` otherwise.

##### 56.4-4 IsRDistributive

`‣ IsRDistributive`( C ) | ( property )

is `true` if the relation ( a + b ) * c = ( a * c ) + ( b * c ) holds for all elements a, b, c in the collection C, and `false` otherwise.

##### 56.4-5 IsDistributive

`‣ IsDistributive`( C ) | ( property )

is `true` if the collection C is both left and right distributive (see `IsLDistributive` (56.4-3), `IsRDistributive` (56.4-4)), and `false` otherwise.

    gap> IsDistributive( Integers );
    true

##### 56.4-6 IsAnticommutative

`‣ IsAnticommutative`( R ) | ( property )

is `true` if the relation a * b = - b * a holds for all elements a, b in the ring R, and `false` otherwise.

##### 56.4-7 IsZeroSquaredRing

`‣ IsZeroSquaredRing`( R ) | ( property )

is `true` if a * a is the zero element of the ring R for all a in R, and `false` otherwise.

##### 56.4-8 IsJacobianRing

`‣ IsJacobianRing`( R ) | ( property )

is `true` if the Jacobi identity holds in the ring R, and `false` otherwise. The Jacobi identity means that x * (y * z) + z * (x * y) + y * (z * x) is the zero element of R, for all elements x, y, z in R.

    gap> L:= FullMatrixLieAlgebra( GF( 5 ), 7 );
    <Lie algebra over GF(5), with 13 generators>
    gap> IsJacobianRing( L );
    true

#### 56.5 Units and Factorizations

##### 56.5-1 IsUnit

`‣ IsUnit`( [R, ]r ) | ( operation )

`IsUnit` returns `true` if r is a unit in the ring R, if given, and otherwise in its default ring (see `DefaultRing` (56.1-3)). If r is not a unit then `false` is returned.

An element r is called a unit in a ring R, if r has an inverse in R.

`IsUnit` may call `Quotient` (56.1-9).

##### 56.5-2 Units

`‣ Units`( R ) | ( attribute )

`Units` returns the group of units of the ring R. This may either be returned as a list or as a group.

An element r is called a unit of a ring R if r has an inverse in R. It is easy to see that the set of units forms a multiplicative group.

    gap> Units( GaussianIntegers );
    [ -1, 1, -E(4), E(4) ]
    gap> Units( GF( 16 ) );
    <group of size 15 with 1 generator>

##### 56.5-3 IsAssociated

`‣ IsAssociated`( [R, ]r, s ) | ( operation )

`IsAssociated` returns `true` if the two ring elements r and s are associated in the ring R, if given, and otherwise in their default ring (see `DefaultRing` (56.1-3)). If the two elements are not associated then `false` is returned.

Two elements r and s of a ring R are called associated if there is a unit u of R such that r u =s.

##### 56.5-4 Associates

`‣ Associates`( [R, ]r ) | ( operation )

`Associates` returns the set of associates of r in the ring R, if given, and otherwise in its default ring (see `DefaultRing` (56.1-3)).

Two elements r and s of a ring R are called associated if there is a unit u of R such that r u = s.

    gap> Associates( Integers, 2 );
    [ -2, 2 ]
    gap> Associates( GaussianIntegers, 2 );
    [ -2, 2, -2*E(4), 2*E(4) ]

##### 56.5-5 StandardAssociate

`‣ StandardAssociate`( [R, ]r ) | ( operation )

`StandardAssociate` returns the standard associate of the ring element r in the ring R, if given, and otherwise in its default ring (see `DefaultRing` (56.1-3)).

The standard associate of a ring element r of R is an associated element of r which is, in a ring dependent way, distinguished among the set of associates of r. For example, in the ring of integers the standard associate is the absolute value.

    gap> x:= Indeterminate( Rationals, "x" );;
    gap> StandardAssociate( -x^2-x+1 );
    x^2+x-1

##### 56.5-6 StandardAssociateUnit

`‣ StandardAssociateUnit`( [R, ]r ) | ( operation )

`StandardAssociateUnit` returns a unit in the ring R such that the ring element r times this unit equals the standard associate of r in R.

If R is not given, the default ring of r is used instead. (see `DefaultRing` (56.1-3)).

    gap> y:= Indeterminate( Rationals, "y" );;
    gap> r:= -y^2-y+1;
    -y^2-y+1
    gap> StandardAssociateUnit( r );
    -1
    gap> StandardAssociateUnit( r ) * r = StandardAssociate( r );
    true

##### 56.5-7 IsIrreducibleRingElement

`‣ IsIrreducibleRingElement`( [R, ]r ) | ( operation )

`IsIrreducibleRingElement` returns `true` if the ring element r is irreducible in the ring R, if given, and otherwise in its default ring (see `DefaultRing` (56.1-3)). If r is not irreducible then `false` is returned.

An element r of a ring R is called irreducible if r is not a unit in R and if there is no nontrivial factorization of r in R, i.e., if there is no representation of r as product s t such that neither s nor t is a unit (see `IsUnit` (56.5-1)). Each prime element (see `IsPrime` (56.5-8)) is irreducible.

    gap> IsIrreducibleRingElement( Integers, 2 );
    true

##### 56.5-8 IsPrime

`‣ IsPrime`( [R, ]r ) | ( operation )

`IsPrime` returns `true` if the ring element r is a prime in the ring R, if given, and otherwise in its default ring (see `DefaultRing` (56.1-3)). If r is not a prime then `false` is returned.

An element r of a ring R is called prime if for each pair s and t such that r divides s t the element r divides either s or t. Note that there are rings where not every irreducible element (see `IsIrreducibleRingElement` (56.5-7)) is a prime.

##### 56.5-9 Factors

`‣ Factors`( [R, ]r ) | ( operation )

`Factors` returns the factorization of the ring element r in the ring R, if given, and otherwise in its default ring (see `DefaultRing` (56.1-3)). The factorization is returned as a list of primes (see `IsPrime` (56.5-8)). Each element in the list is a standard associate (see `StandardAssociate` (56.5-5)) except the first one, which is multiplied by a unit as necessary to have `Product( Factors( R, r ) ) = r`. This list is usually also sorted, thus smallest prime factors come first. If r is a unit or zero, `Factors( R, r ) = [ r ]`.

    gap> x:= Indeterminate( GF(2), "x" );;
    gap> pol:= x^2+x+1;
    x^2+x+Z(2)^0
    gap> Factors( pol );
    [ x^2+x+Z(2)^0 ]
    gap> Factors( PolynomialRing( GF(4) ), pol );
    [ x+Z(2^2), x+Z(2^2)^2 ]

##### 56.5-10 PadicValuation

`‣ PadicValuation`( r, p ) | ( operation )

`PadicValuation` is the operation to compute the p-adic valuation of a ring element r.

#### 56.6 Euclidean Rings

##### 56.6-1 IsEuclideanRing

`‣ IsEuclideanRing`( R ) | ( category )

A ring R is called a Euclidean ring if it is a non-trivial commutative ring and there exists a function δ, called the Euclidean degree, from R-{0_R} into a well-ordered set (such as the nonnegative integers), such that for every pair r ∈ R and s ∈ R-{0_R} there exists an element q such that either r - q s = 0_R or δ(r - q s) < δ( s ). In GAP the Euclidean degree δ is implicitly built into a ring and cannot be changed. The existence of this division with remainder implies that the Euclidean algorithm can be applied to compute a greatest common divisor of two elements, which in turn implies that R is a unique factorization ring.

    gap> IsEuclideanRing( GaussianIntegers );
    true

##### 56.6-2 EuclideanDegree

`‣ EuclideanDegree`( [R, ]r ) | ( operation )

`EuclideanDegree` returns the Euclidean degree of the ring element r in the ring R, if given, and otherwise in its default ring (see `DefaultRing` (56.1-3)).

The ring R must be a Euclidean ring (see `IsEuclideanRing` (56.6-1)).

    gap> EuclideanDegree( GaussianIntegers, 3 );
    9

##### 56.6-3 EuclideanQuotient

`‣ EuclideanQuotient`( [R, ]r, m ) | ( operation )

`EuclideanQuotient` returns the Euclidean quotient of the ring elements r and m in the ring R, if given, and otherwise in their default ring (see `DefaultRing` (56.1-3)).

The ring R must be a Euclidean ring (see `IsEuclideanRing` (56.6-1)), otherwise an error is signalled.

    gap> EuclideanQuotient( 8, 3 );
    2

##### 56.6-4 EuclideanRemainder

`‣ EuclideanRemainder`( [R, ]r, m ) | ( operation )

`EuclideanRemainder` returns the Euclidean remainder of the ring element r modulo the ring element m in the ring R, if given, and otherwise in their default ring (see `DefaultRing` (56.1-3)).

The ring R must be a Euclidean ring (see `IsEuclideanRing` (56.6-1)), otherwise an error is signalled.

    gap> EuclideanRemainder( 8, 3 );
    2

##### 56.6-5 QuotientRemainder

`‣ QuotientRemainder`( [R, ]r, m ) | ( operation )

`QuotientRemainder` returns the Euclidean quotient and the Euclidean remainder of the ring elements r and m in the ring R, if given, and otherwise in their default ring (see `DefaultRing` (56.1-3)). The result is a pair of ring elements.

The ring R must be a Euclidean ring (see `IsEuclideanRing` (56.6-1)), otherwise an error is signalled.

    gap> QuotientRemainder( GaussianIntegers, 8, 3 );
    [ 3, -1 ]

#### 56.7 Gcd and Lcm

##### 56.7-1 Gcd

`‣ Gcd`( [R, ]r1, r2, ... ) | ( function )

`‣ Gcd`( [R, ]list ) | ( function )

`Gcd` returns the greatest common divisor of the ring elements r1, r2, ... resp. of the ring elements in the list list in the ring R, if given, and otherwise in their default ring, see `DefaultRing` (56.1-3).

`Gcd` returns the standard associate (see `StandardAssociate` (56.5-5)) of the greatest common divisors.

A divisor of an element r in the ring R is an element d∈ R such that r is a multiple of d. A common divisor of the elements r_1, r_2, ... in the ring R is an element d∈ R which is a divisor of each r_1, r_2, .... A greatest common divisor d in addition has the property that every other common divisor of r_1, r_2, ... is a divisor of d.

Note that this in particular implies the following: For the zero element z of R, we have `Gcd( r,` z`) = Gcd(` z`, r ) = StandardAssociate( r )` and `Gcd(` z`,` z`) =` z.

    gap> Gcd( Integers, [ 10, 15 ] );
    5

##### 56.7-2 GcdOp

`‣ GcdOp`( [R, ]r, s ) | ( operation )

`GcdOp` is the operation to compute the greatest common divisor of two ring elements r, s in the ring R or in their default ring.

##### 56.7-3 GcdRepresentation

`‣ GcdRepresentation`( [R, ]r1, r2, ... ) | ( function )

`‣ GcdRepresentation`( [R, ]list ) | ( function )

`GcdRepresentation` returns a representation of the greatest common divisor of the ring elements r1, r2, ... resp. of the ring elements in the list list in the Euclidean ring R, if given, and otherwise in their default ring, see `DefaultRing` (56.1-3).

A representation of the gcd g of the elements r_1, r_2, ... of a ring R is a list of ring elements s_1, s_2, ... of R, such that g = s_1 r_1 + s_2 r_2 + ⋯. Such representations do not exist in all rings, but they do exist in Euclidean rings (see `IsEuclideanRing` (56.6-1)), which can be shown using the Euclidean algorithm, which in fact can compute those coefficients.

    gap> a:= Indeterminate( Rationals, "a" );;
    gap> GcdRepresentation( a^2+1, a^3+1 );
    [ -1/2*a^2-1/2*a+1/2, 1/2*a+1/2 ]

`Gcdex` (14.3-5) provides similar functionality over the integers.

##### 56.7-4 GcdRepresentationOp

`‣ GcdRepresentationOp`( [R, ]r, s ) | ( operation )

`GcdRepresentationOp` is the operation to compute the representation of the greatest common divisor of two ring elements r, s in the Euclidean ring R or in their default ring, respectively.

##### 56.7-5 ShowGcd

`‣ ShowGcd`( a, b ) | ( function )

This function takes two elements a and b of an Euclidean ring and returns their greatest common divisor. It will print out the steps performed by the Euclidean algorithm, as well as the rearrangement of these steps to express the gcd as a ring combination of a and b.

    gap> ShowGcd(192,42);
    192=4*42 + 24
    42=1*24 + 18
    24=1*18 + 6
    18=3*6 + 0
    The Gcd is 6
     = 1*24 -1*18
     = -1*42 + 2*24
     = 2*192 -9*42
    6

##### 56.7-6 Lcm

`‣ Lcm`( [R, ]r1, r2, ... ) | ( function )

`‣ Lcm`( [R, ]list ) | ( function )

`Lcm` returns the least common multiple of the ring elements r1, r2, ... resp. of the ring elements in the list list in the ring R, if given, and otherwise in their default ring, see `DefaultRing` (56.1-3).

`Lcm` returns the standard associate (see `StandardAssociate` (56.5-5)) of the least common multiples.

A least common multiple of the elements r_1, r_2, ... of the ring R is an element m that is a multiple of r_1, r_2, ..., and every other multiple of these elements is a multiple of m.

Note that this in particular implies the following: For the zero element z of R, we have `Lcm( r,` z`) = Lcm(` z`, r ) = StandardAssociate( r )` and `Lcm(` z`,` z`) =` z.

##### 56.7-7 LcmOp

`‣ LcmOp`( [R, ]r, s ) | ( operation )

`LcmOp` is the operation to compute the least common multiple of two ring elements r, s in the ring R or in their default ring, respectively.

The default methods for this uses the equality lcm( m, n ) = m*n / gcd( m, n ) (see `GcdOp` (56.7-2)).

##### 56.7-8 QuotientMod

`‣ QuotientMod`( [R, ]r, s, m ) | ( operation )

`QuotientMod` returns a quotient of the ring elements r and s modulo the ring element m in the ring R, if given, and otherwise in their default ring, see `DefaultRing` (56.1-3).

R must be a Euclidean ring (see `IsEuclideanRing` (56.6-1)) so that `EuclideanRemainder` (56.6-4) can be applied. If no modular quotient exists, `fail` is returned.

A quotient q of r and s modulo m is an element of R such that q s = r modulo m, i.e., such that q s - r is divisible by m in R and that q is either zero (if r is divisible by m) or the Euclidean degree of q is strictly smaller than the Euclidean degree of m.

    gap> QuotientMod( 7, 2, 3 );
    2

##### 56.7-9 PowerMod

`‣ PowerMod`( [R, ]r, e, m ) | ( operation )

`PowerMod` returns the e-th power of the ring element r modulo the ring element m in the ring R, if given, and otherwise in their default ring, see `DefaultRing` (56.1-3). e must be an integer.

R must be a Euclidean ring (see `IsEuclideanRing` (56.6-1)) so that `EuclideanRemainder` (56.6-4) can be applied to its elements.

If e is positive the result is r`^`e modulo m. If e is negative then `PowerMod` first tries to find the inverse of r modulo m, i.e., i such that i r = 1 modulo m. If the inverse does not exist an error is signalled. If the inverse does exist `PowerMod` returns `PowerMod( R, i, -e, m )`.

`PowerMod` reduces the intermediate values modulo m, improving performance drastically when e is large and m small.

    gap> PowerMod( 12, 100000, 7 );
    2

##### 56.7-10 InterpolatedPolynomial

`‣ InterpolatedPolynomial`( R, x, y ) | ( operation )

`InterpolatedPolynomial` returns, for given lists x, y of elements in a ring R of the same length n the unique polynomial of degree less than n which has value y[i] at x[i], for all i ∈ { 1, ..., n }. Note that the elements in x must be distinct.

    gap> InterpolatedPolynomial( Integers, [ 1, 2, 3 ], [ 5, 7, 0 ] );
    -9/2*x^2+31/2*x-6

#### 56.8 Homomorphisms of Rings

A ring homomorphism is a mapping between two rings that respects addition and multiplication.

Currently GAP supports ring homomorphisms between finite rings (using straightforward methods) and ring homomorphisms with additional structures, where source and range are in fact algebras and where also the linear structure is respected, see 62.10.

##### 56.8-1 RingGeneralMappingByImages

`‣ RingGeneralMappingByImages`( R, S, gens, imgs ) | ( operation )

is a general mapping from the ring A to the ring S. This general mapping is defined by mapping the entries in the list gens (elements of R) to the entries in the list imgs (elements of S), and taking the additive and multiplicative closure.

gens need not generate R as a ring, and if the specification does not define an additive and multiplicative mapping then the result will be multivalued. Hence, in general it is not a mapping.

##### 56.8-2 RingHomomorphismByImages

`‣ RingHomomorphismByImages`( R, S, gens, imgs ) | ( function )

`RingHomomorphismByImages` returns the ring homomorphism with source R and range S that is defined by mapping the list gens of generators of R to the list imgs of images in S.

If gens does not generate R or if the homomorphism does not exist (i.e., if mapping the generators describes only a multi-valued mapping) then `fail` is returned.

One can avoid the checks by calling `RingHomomorphismByImagesNC` (56.8-3), and one can construct multi-valued mappings with `RingGeneralMappingByImages` (56.8-1).

##### 56.8-3 RingHomomorphismByImagesNC

`‣ RingHomomorphismByImagesNC`( R, S, gens, imgs ) | ( operation )

`RingHomomorphismByImagesNC` is the operation that is called by the function `RingHomomorphismByImages` (56.8-2). Its methods may assume that gens generates R as a ring and that the mapping of gens to imgs defines a ring homomorphism. Results are unpredictable if these conditions do not hold.

For creating a possibly multi-valued mapping from R to S that respects addition and multiplication, `RingGeneralMappingByImages` (56.8-1) can be used.

##### 56.8-4 NaturalHomomorphismByIdeal

`‣ NaturalHomomorphismByIdeal`( R, I ) | ( operation )

is the homomorphism of rings provided by the natural projection map of R onto the quotient ring R/I. This map can be used to take pre-images in the original ring from elements in the quotient.

#### 56.9 Small Rings

GAP contains a library of small (order up to 15) rings.

##### 56.9-1 SmallRing

`‣ SmallRing`( s, n ) | ( function )

returns the n-th ring of order s from a library of rings of small order (up to isomorphism).

    gap> R:=SmallRing(8,37);
    <ring with 3 generators>
    gap> ShowMultiplicationTable(R);
    *     | 0*a   c     b     b+c   a     a+c   a+b   a+b+c
    ------+------------------------------------------------
    0*a   | 0*a   0*a   0*a   0*a   0*a   0*a   0*a   0*a
    c     | 0*a   0*a   0*a   0*a   0*a   0*a   0*a   0*a
    b     | 0*a   0*a   0*a   0*a   b     b     b     b
    b+c   | 0*a   0*a   0*a   0*a   b     b     b     b
    a     | 0*a   c     b     b+c   a+b   a+b+c a     a+c
    a+c   | 0*a   c     b     b+c   a+b   a+b+c a     a+c
    a+b   | 0*a   c     b     b+c   a     a+c   a+b   a+b+c
    a+b+c | 0*a   c     b     b+c   a     a+c   a+b   a+b+c

##### 56.9-2 NumberSmallRings

`‣ NumberSmallRings`( s ) | ( function )

returns the number of (nonisomorphic) rings of order s stored in the library of small rings.

    gap> List([1..15],NumberSmallRings);
    [ 1, 2, 2, 11, 2, 4, 2, 52, 11, 4, 2, 22, 2, 4, 4 ]

##### 56.9-3 Subrings

`‣ Subrings`( R ) | ( attribute )

for a finite ring R this function returns a list of all subrings of R.

    gap> Subrings(SmallRing(8,37));
    [ <ring with 1 generator>, <ring with 1 generator>,
      <ring with 1 generator>, <ring with 1 generator>,
      <ring with 1 generator>, <ring with 1 generator>,
      <ring with 2 generators>, <ring with 2 generators>,
      <ring with 2 generators>, <ring with 2 generators>,
      <ring with 3 generators> ]

##### 56.9-4 Ideals

`‣ Ideals`( R ) | ( attribute )

for a finite ring R this function returns a list of all ideals of R.

    gap> Ideals(SmallRing(8,37));
    [ <ring with 1 generator>, <ring with 1 generator>,
      <ring with 1 generator>, <ring with 2 generators>,
      <ring with 3 generators> ]

##### 56.9-5 DirectSum

`‣ DirectSum`( R{, S} ) | ( function )

`‣ DirectSumOp`( list, expl ) | ( operation )

These functions construct the direct sum of the rings given as arguments. `DirectSum` takes an arbitrary positive number of arguments and calls the operation `DirectSumOp`, which takes exactly two arguments, namely a nonempty list of rings and one of these rings. (This somewhat strange syntax allows the method selection to choose a reasonable method for special cases.)

    gap> DirectSum(SmallRing(5,1),SmallRing(5,1));
    <ring with 2 generators>

##### 56.9-6 RingByStructureConstants

`‣ RingByStructureConstants`( moduli, sctable[, nameinfo] ) | ( function )

returns a ring R whose additive group is described by the list moduli, with multiplication defined by the structure constants table sctable. The optional argument nameinfo can be used to prescribe names for the elements of the canonical generators of R; it can be either a string name (then name `1`, name `2` etc. are chosen) or a list of strings which are then chosen.

    gap> T:=EmptySCTable( 1, 0 );;
    gap> SetEntrySCTable( T, 1, 1, [ 1, 1 ] );
    gap> R:=RingByStructureConstants([9], T);   # Z/9Z
    <ring with 1 generator>
    gap> Elements(R);
    [ 0*r.1, r.1, 2*r.1, 3*r.1, 4*r.1, 5*r.1, 6*r.1, 7*r.1, -r.1 ]

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