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### 7. Ideals of numerical semigroups

#### 7.1 Ideals of numerical semigroups

Let S be a numerical semigroup. A set I of integers is an ideal relative to a numerical semigroup S provided that I+Ssubseteq I and that there exists din S such that d+Isubseteq S.

If i_1,...,i_k is a subset of Z, then the set I=i_1,...,i_k+S=bigcup_n=1^k i_n+S is an ideal relative to S, and i_1,..., i_k is a system of generators of I. A system of generators M is minimal if no proper subset of M generates the same ideal. Usually, ideals are specified by means of its generators and the ambient numerical semigroup to which they are ideals (for more information see for instance [VF97]).

##### 7.1-1 IdealOfNumericalSemigroup

`> IdealOfNumericalSemigroup`( l, S ) | ( function )

S is a numerical semigroup and l a list of integers.

The output is the ideal of S generated by l

There are several shortcuts for this function, as shown in the example.

    gap> IdealOfNumericalSemigroup([3,5],NumericalSemigroup(9,11));
    <Ideal of numerical semigroup>
    gap> [3,5]+NumericalSemigroup(9,11);
    <Ideal of numerical semigroup>
    gap> last=last2;
    true
    gap> 3+NumericalSemigroup(5,9);
    <Ideal of numerical semigroup>

##### 7.1-2 IsIdealOfNumericalSemigroup

`> IsIdealOfNumericalSemigroup`( Obj ) | ( function )

Tests if the object Obj is an ideal of a numerical semigroup.

    gap> I:=[1..7]+NumericalSemigroup(7,19);
    <Ideal of numerical semigroup>
    gap> IsIdealOfNumericalSemigroup(I);
    true
    gap> IsIdealOfNumericalSemigroup(2);
    false

##### 7.1-3 MinimalGeneratingSystemOfIdealOfNumericalSemigroup

`> MinimalGeneratingSystemOfIdealOfNumericalSemigroup`( I ) | ( function )

I is an ideal of a numerical semigroup.

The output is the minimal system of generators of I.

    gap> I:=[3,5,9]+NumericalSemigroup(2,11);
    <Ideal of numerical semigroup>
    gap> MinimalGeneratingSystemOfIdealOfNumericalSemigroup(I);
    [ 3 ]

##### 7.1-4 GeneratorsOfIdealOfNumericalSemigroup

`> GeneratorsOfIdealOfNumericalSemigroup`( I ) | ( function )

`> GeneratorsOfIdealOfNumericalSemigroupNC`( I ) | ( function )

I is an ideal of a numerical semigroup.

The output of `GeneratorsOfIdealOfNumericalSemigroup` is a system of generators of the ideal. If the minimal system of generators is known, then it is used as output. `GeneratorsOfIdealOfNumericalSemigroupNC` always returns the set of generators stored in I!.generators.

    gap> I:=[3,5,9]+NumericalSemigroup(2,11);
    <Ideal of numerical semigroup>
    gap> GeneratorsOfIdealOfNumericalSemigroup(I);
    [ 3, 5, 9 ]
    gap> MinimalGeneratingSystemOfIdealOfNumericalSemigroup(I);
    [ 3 ]
    gap> GeneratorsOfIdealOfNumericalSemigroup(I);
    [ 3 ]
    gap> GeneratorsOfIdealOfNumericalSemigroupNC(I);
    [ 3, 5, 9 ]

##### 7.1-5 AmbientNumericalSemigroupOfIdeal

`> AmbientNumericalSemigroupOfIdeal`( I ) | ( function )

I is an ideal of a numerical semigroup, say S.

The output is S.

    gap> I:=[3,5,9]+NumericalSemigroup(2,11);
    <Ideal of numerical semigroup>
    gap> AmbientNumericalSemigroupOfIdeal(I);
    <Numerical semigroup with 2 generators>

##### 7.1-6 SmallElementsOfIdealOfNumericalSemigroup

`> SmallElementsOfIdealOfNumericalSemigroup`( I ) | ( function )

I is an ideal of a numerical semigroup.

The output is a list with the elements in I that are less than or equal to the greatest integer not belonging to the ideal plus one.

    gap> I:=[3,5,9]+NumericalSemigroup(2,11);
    <Ideal of numerical semigroup>
    gap> SmallElementsOfIdealOfNumericalSemigroup(I);
    [ 3, 5, 7, 9, 11, 13 ]
    gap> J:=[2,11]+NumericalSemigroup(2,11);
    <Ideal of numerical semigroup>
    gap> SmallElementsOfIdealOfNumericalSemigroup(J);
    [ 2, 4, 6, 8, 10 ]

##### 7.1-7 BelongsToIdealOfNumericalSemigroup

`> BelongsToIdealOfNumericalSemigroup`( n, I ) | ( function )

I is an ideal of a numerical semigroup, n is an integer.

The output is true if n belongs to I.

n in I can be used for short.

    gap> J:=[2,11]+NumericalSemigroup(2,11);
    <Ideal of numerical semigroup>
    gap> BelongsToIdealOfNumericalSemigroup(9,J);
    false
    gap> 9 in J;
    false
    gap> BelongsToIdealOfNumericalSemigroup(10,J);
    true
    gap> 10 in J;
    true

##### 7.1-8 SumIdealsOfNumericalSemigroup

`> SumIdealsOfNumericalSemigroup`( I, J ) | ( function )

I, J are ideals of a numerical semigroup.

The output is the sum of both ideals i+j | iin I, jin J.

I + J is a synonym of this function.

    gap> I:=[3,5,9]+NumericalSemigroup(2,11);
    <Ideal of numerical semigroup>
    gap> J:=[2,11]+NumericalSemigroup(2,11);
    <Ideal of numerical semigroup>
    gap> I+J;
    <Ideal of numerical semigroup>
    gap> MinimalGeneratingSystemOfIdealOfNumericalSemigroup(last);
    [ 5, 14 ]
    gap> SumIdealsOfNumericalSemigroup(I,J);
    <Ideal of numerical semigroup>
    gap> MinimalGeneratingSystemOfIdealOfNumericalSemigroup(last);
    [ 5, 14 ]

##### 7.1-9 MultipleOfIdealOfNumericalSemigroup

`> MultipleOfIdealOfNumericalSemigroup`( n, I ) | ( function )

I is an ideal of a numerical semigroup, n is a non negative integer.

The output is the ideal I+cdots+I (n times).

n * I can be used for short.

    gap> I:=[0,1]+NumericalSemigroup(3,5,7);
    <Ideal of numerical semigroup>
    gap> MinimalGeneratingSystemOfIdealOfNumericalSemigroup(2*I);
    [ 0, 1, 2 ]

##### 7.1-10 SubtractIdealsOfNumericalSemigroup

`> SubtractIdealsOfNumericalSemigroup`( I, J ) | ( function )

I, J are ideals of a numerical semigroup.

The output is the ideal zin Z | z+Jsubseteq I.

I - J is a synonym of this function. The following example appears in [HS04].

    gap> S:=NumericalSemigroup(14, 15, 20, 21, 25);
    <Numerical semigroup with 5 generators>
    gap> I:=[0,1]+S;
    <Ideal of numerical semigroup>
    gap> II:=(0+S)-I;
    <Ideal of numerical semigroup>
    gap> MinimalGeneratingSystemOfIdealOfNumericalSemigroup(I);
    [ 0, 1 ]
    gap> MinimalGeneratingSystemOfIdealOfNumericalSemigroup(II);
    [ 14, 20 ]
    gap> MinimalGeneratingSystemOfIdealOfNumericalSemigroup(I+II);
    [ 14, 15, 20, 21 ]

##### 7.1-11 DifferenceOfIdealsOfNumericalSemigroup

`> DifferenceOfIdealsOfNumericalSemigroup`( I, J ) | ( function )

I, J are ideals of a numerical semigroup. J must be contained in I.

The output is the set I\ J.

    gap> S:=NumericalSemigroup(14, 15, 20, 21, 25);
    <Numerical semigroup with 5 generators>
    gap> I:=[0,1]+S;
    <Ideal of numerical semigroup>
    gap> 2*I-2*I;
    <Ideal of numerical semigroup>
    gap> I-I;
    <Ideal of numerical semigroup>
    gap> DifferenceOfIdealsOfNumericalSemigroup(last2,last);
    [ 26, 27, 37, 38 ]

##### 7.1-12 TranslationOfIdealOfNumericalSemigroup

`> TranslationOfIdealOfNumericalSemigroup`( k, I ) | ( function )

Given an ideal I of a numerical semigroup S and an integer k returns an ideal of the numerical semigroup S generated by i_1+k,...,i_n+k where i_1,...,i_n is the system of generators of I.

As a synonym to `TranslationOfIdealOfNumericalSemigroup(k, I)` the expression `k + I` may be used.

    gap> s:=NumericalSemigroup(13,23);
    <Numerical semigroup with 2 generators>
    gap> l:=List([1..6], _ -> Random([8..34]));
    [ 22, 29, 34, 25, 10, 12 ]
    gap> I:=IdealOfNumericalSemigroup(l, s);
    <Ideal of numerical semigroup>
    gap> It:=TranslationOfIdealOfNumericalSemigroup(7,I);
    <Ideal of numerical semigroup>
    gap> It2:=7+I;
    <Ideal of numerical semigroup>
    gap> It2=It;
    true

##### 7.1-13 HilbertFunctionOfIdealOfNumericalSemigroup

`> HilbertFunctionOfIdealOfNumericalSemigroup`( n, I ) | ( function )

I is an ideal of a numerical semigroup, n is a non negative integer. I must be contained in its ambient semigroup.

The output is the cardinality of the set n I\ (n+1)I.

    gap> I:=[6,9,11]+NumericalSemigroup(6,9,11);;
    gap> List([1..7],n->HilbertFunctionOfIdealOfNumericalSemigroup(n,I));
    [ 3, 11, 24, 48, 96, 192, 384 ]

##### 7.1-14 BlowUpIdealOfNumericalSemigroup

`> BlowUpIdealOfNumericalSemigroup`( I ) | ( function )

I is an ideal of a numerical semigroup.

The output is the ideal bigcup_n>= 0 nI-nI.

    gap> I:=[0,2]+NumericalSemigroup(6,9,11);;
    gap> BlowUpIdealOfNumericalSemigroup(I);;
    gap> SmallElementsOfIdealOfNumericalSemigroup(last);
    [ 0, 2, 4, 6, 8 ]

##### 7.1-15 ReductionNumberIdealNumericalSemigroup

`> ReductionNumberIdealNumericalSemigroup`( I ) | ( function )

I is an ideal of a numerical semigroup.

The output is the least integer such that n I + i=(n+1)I, where i=min(I).

    gap> I:=[0,2]+NumericalSemigroup(6,9,11);;
    gap> ReductionNumberIdealNumericalSemigroup(I);
    2

##### 7.1-16 MaximalIdealOfNumericalSemigroup

`> MaximalIdealOfNumericalSemigroup`( S ) | ( function )

Returns the maximal ideal of the numerical semigroup S.

    gap> MaximalIdealOfNumericalSemigroup(NumericalSemigroup(3,7));
    <Ideal of numerical semigroup>

##### 7.1-17 BlowUpOfNumericalSemigroup

`> BlowUpOfNumericalSemigroup`( S ) | ( function )

If M is the maximal ideal of the numerical semigroup, then the output is the numerical semigroup bigcup_n>= 0 nM-nM.

    gap> s:=NumericalSemigroup(30, 35, 42, 47, 148, 153, 157, 169, 181, 193);
    <Numerical semigroup with 10 generators>
    gap> BlowUpOfNumericalSemigroup(s);
    <Numerical semigroup with 10 generators>
    gap> SmallElementsOfNumericalSemigroup(last);
    [ 0, 5, 10, 12, 15, 17, 20, 22, 24, 25, 27, 29, 30, 32, 34, 35, 36, 37, 39,
      40, 41, 42, 44 ]
    gap> m:=MaximalIdealOfNumericalSemigroup(s);
    <Ideal of numerical semigroup>
    gap> BlowUpIdealOfNumericalSemigroup(m);
    <Ideal of numerical semigroup>
    gap> SmallElementsOfIdealOfNumericalSemigroup(last);
    [ 0, 5, 10, 12, 15, 17, 20, 22, 24, 25, 27, 29, 30, 32, 34, 35, 36, 37, 39,
      40, 41, 42, 44 ]

##### 7.1-18 MicroInvariantsOfNumericalSemigroup

`> MicroInvariantsOfNumericalSemigroup`( S ) | ( function )

Returns the microinvariants of the numerical semigroup S defined in [E01]. For their computation we have used the formula given in [BF06]. The Ap\'ery set of S and its blow up are involved in this computation.

    gap> s:=NumericalSemigroup(30, 35, 42, 47, 148, 153, 157, 169, 181, 193);
    <Numerical semigroup with 10 generators>
    gap> bu:=BlowUpOfNumericalSemigroup(s);
    <Numerical semigroup with 10 generators>
    gap> ap:=AperyListOfNumericalSemigroupWRTElement(s,30);;
    gap> apbu:=AperyListOfNumericalSemigroupWRTElement(bu,30);;
    gap> (ap-apbu)/30;
    [ 0, 4, 4, 3, 2, 1, 3, 4, 4, 3, 2, 3, 1, 4, 4, 3, 3, 1, 4, 4, 4, 3, 2, 4, 2,
      5, 4, 3, 3, 2 ]
    gap> MicroInvariantsOfNumericalSemigroup(s)=last;
    true

##### 7.1-19 IsGradedAssociatedRingNumericalSemigroupCM

`> IsGradedAssociatedRingNumericalSemigroupCM`( S ) | ( function )

Returns true if the graded ring associated to K[[S]] is Cohen-Macaulay, and false otherwise. This test is the implementation of the algorithm given in [BF06].

    gap> s:=NumericalSemigroup(30, 35, 42, 47, 148, 153, 157, 169, 181, 193);
    <Numerical semigroup with 10 generators>
    gap> IsGradedAssociatedRingNumericalSemigroupCM(s);
    false
    gap> MicroInvariantsOfNumericalSemigroup(s);
    [ 0, 4, 4, 3, 2, 1, 3, 4, 4, 3, 2, 3, 1, 4, 4, 3, 3, 1, 4, 4, 4, 3, 2, 4, 2,
      5, 4, 3, 3, 2 ]
    gap> List(AperyListOfNumericalSemigroupWRTElement(s,30),
    > w->MaximumDegreeOfElementWRTNumericalSemigroup (w,s));
    [ 0, 1, 4, 1, 2, 1, 3, 1, 4, 3, 2, 3, 1, 1, 4, 3, 3, 1, 4, 1, 4, 3, 2, 4, 2,
    5, 4, 3, 1, 2 ]
    gap> last=last2;
    false
    gap> s:=NumericalSemigroup(4,6,11);
    <Numerical semigroup with 3 generators>
    gap> IsGradedAssociatedRingNumericalSemigroupCM(s);
    true
    gap> MicroInvariantsOfNumericalSemigroup(s);
    [ 0, 2, 1, 1 ]
    gap> List(AperyListOfNumericalSemigroupWRTElement(s,4),
    > w->MaximumDegreeOfElementWRTNumericalSemigroup(w,s));
    [ 0, 2, 1, 1 ]

##### 7.1-20 CanonicalIdealOfNumericalSemigroup

`> CanonicalIdealOfNumericalSemigroup`( S ) | ( function )

Computes a canonical ideal of S ([BF06]): x in Z | g-x not in S.

    gap> s:=NumericalSemigroup(4,6,11);
    <Numerical semigroup with 3 generators>
    gap> m:=MaximalIdealOfNumericalSemigroup(s);
    <Ideal of numerical semigroup>
    gap> c:=CanonicalIdealOfNumericalSemigroup(s);
    <Ideal of numerical semigroup>
    gap> (m-c)-c=m;
    true
    gap> id:=3+s;
    <Ideal of numerical semigroup>
    gap> (id-c)-c=id;
    true

##### 7.1-21 IntersectionIdealsOfNumericalSemigroup

`> IntersectionIdealsOfNumericalSemigroup`( I, J ) | ( function )

Given two ideals I and J of a numerical semigroup S returns the ideal of the numerical semigroup S which is the intersection of the ideals I and J.

    gap> i:=IdealOfNumericalSemigroup([75,89],s);
    <Ideal of numerical semigroup>
    gap> j:=IdealOfNumericalSemigroup([115,289],s);
    <Ideal of numerical semigroup>
    gap> IntersectionIdealsOfNumericalSemigroup(i,j);
    <Ideal of numerical semigroup>

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