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### 5 Constructing numerical semigroups from others

#### 5.1 Adding and removing elements of a numerical semigroup

In this section we show how to construct new numerical semigroups from a given numerical semigroup. Two dual operations are presented. The first one removes a minimal generator from a numerical semigroup. The second adds a special gap to a semigroup (see [RGGJ03]).

##### 5.1-1 RemoveMinimalGeneratorFromNumericalSemigroup
 `‣ RemoveMinimalGeneratorFromNumericalSemigroup`( n, S ) ( function )

S is a numerical semigroup and n is one if its minimal generators.

The output is the numerical semigroup S ∖{n} (see [RGGJ03]; S∖{n} is a numerical semigroup if and only if n is a minimal generator of S).

```gap> s:=NumericalSemigroup(3,5,7);
<Numerical semigroup with 3 generators>
gap> RemoveMinimalGeneratorFromNumericalSemigroup(7,s);
<Numerical semigroup with 3 generators>
gap> MinimalGeneratingSystemOfNumericalSemigroup(last);
[ 3, 5 ]

```

 `‣ AddSpecialGapOfNumericalSemigroup`( g, S ) ( function )

S is a numerical semigroup and g is a special gap of S

The output is the numerical semigroup S ∪{g} (see [RGGJ03], where it is explained why this set is a numerical semigroup).

```gap> s:=NumericalSemigroup(3,5,7);
<Numerical semigroup with 3 generators>
gap> s2:=RemoveMinimalGeneratorFromNumericalSemigroup(5,s);
<Numerical semigroup with 3 generators>
<Numerical semigroup>
gap> SmallElementsOfNumericalSemigroup(s) =
> SmallElementsOfNumericalSemigroup(s3);
true
gap> s=s3;
true

```

##### 5.1-3 IntersectionOfNumericalSemigroups
 `‣ IntersectionOfNumericalSemigroups`( S, T ) ( function )

S and T are numerical semigroups. Computes the intersection of S and T (which is a numerical semigroup).

```gap> S := NumericalSemigroup("modular", 5,53);
<Modular numerical semigroup satisfying 5x mod 53 <= x >
gap> T := NumericalSemigroup(2,17);
<Numerical semigroup with 2 generators>
gap> SmallElementsOfNumericalSemigroup(S);
[ 0, 11, 12, 13, 22, 23, 24, 25, 26, 32, 33, 34, 35, 36, 37, 38, 39, 43 ]
gap> SmallElementsOfNumericalSemigroup(T);
[ 0, 2, 4, 6, 8, 10, 12, 14, 16 ]
gap> IntersectionOfNumericalSemigroups(S,T);
<Numerical semigroup>
gap> SmallElementsOfNumericalSemigroup(last);
[ 0, 12, 22, 23, 24, 25, 26, 32, 33, 34, 35, 36, 37, 38, 39, 43 ]
```

##### 5.1-4 QuotientOfNumericalSemigroup
 `‣ QuotientOfNumericalSemigroup`( S, n ) ( function )

S is a numerical semigroup and n is an integer. Computes the quotient of S by n, that is, the set { x∈ N | nx ∈ S}, which is again a numerical semigroup. `S / n` may be used as a short for `QuotientOfNumericalSemigroup(S, n)`.

```gap> s:=NumericalSemigroup(3,29);
<Numerical semigroup with 2 generators>
gap> SmallElementsOfNumericalSemigroup(s);
[ 0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 29, 30, 32, 33, 35, 36, 38,
39, 41, 42, 44, 45, 47, 48, 50, 51, 53, 54, 56 ]
gap> t:=QuotientOfNumericalSemigroup(s,7);
<Numerical semigroup>
gap> SmallElementsOfNumericalSemigroup(t);
[ 0, 3, 5, 6, 8 ]
gap> u := s / 7;
<Numerical semigroup>
gap> SmallElementsOfNumericalSemigroup(u);
[ 0, 3, 5, 6, 8 ]
```

#### 5.2 Constructing the set of all numerical semigroups containing a given numerical semigroup

In order to construct the set of numerical semigroups containing a fixed numerical semigroup S, one first constructs its unitary extensions, that is to say, the sets S∪{g} that are numerical semigroups with g a positive integer. This is achieved by constructing the special gaps of the semigroup, and then adding each of them to the numerical semigroup. Then we repeat the process for each of this new numerical semigroups until we reach N.

These procedures are described in [RGGJ03].

##### 5.2-1 OverSemigroupsNumericalSemigroup
 `‣ OverSemigroupsNumericalSemigroup`( s ) ( function )

s is a numerical semigroup. The output is the set of numerical semigroups containing it.

```gap> OverSemigroupsNumericalSemigroup(NumericalSemigroup(3,5,7));
[ <Numerical semigroup with 1 generators>, <Numerical semigroup>,
<Numerical semigroup>, <Numerical semigroup with 3 generators> ]
gap> List(last,s->MinimalGeneratingSystemOfNumericalSemigroup(s));
[ [ 1 ], [ 3, 4, 5 ], [ 2, 3 ], [ 3, 5, 7 ] ]
```

##### 5.2-2 NumericalSemigroupsWithFrobeniusNumber
 `‣ NumericalSemigroupsWithFrobeniusNumber`( f ) ( function )

f is an non zero integer greater than or equal to -1. The output is the set of numerical semigroups with Frobenius number f. The algorithm implemented is given in [RGGM04].

```gap> Length(NumericalSemigroupsWithFrobeniusNumber(20));
900

```

Given a numerical semigroup of genus g, removing minimal generators, one obtains numerical semigroups of genus g+1. In order to avoid repetitions, we only remove minimal generators greater than the frobenius number of the numerical semigroup (this is accomplished with the local function sons).

These procedures are described in [RGGB03] and [Bra08].

##### 5.2-3 NumericalSemigroupsWithGenus
 `‣ NumericalSemigroupsWithGenus`( g ) ( function )

g is a nonnegative integer. The output is the set of numerical semigroups with genusg .

```gap> NumericalSemigroupsWithGenus(5);
[ <Proportionally modular numerical semigroup satisfying 11x mod 66 <= 5x >,
<Numerical semigroup with 5 generators>,
<Numerical semigroup with 5 generators>,
<Numerical semigroup with 5 generators>,
<Numerical semigroup with 5 generators>,
<Numerical semigroup with 4 generators>,
<Numerical semigroup with 4 generators>,
<Numerical semigroup with 4 generators>,
<Numerical semigroup with 4 generators>,
<Numerical semigroup with 3 generators>,
<Numerical semigroup with 3 generators>,
<Numerical semigroup with 2 generators> ]
gap> List(last,MinimalGeneratingSystemOfNumericalSemigroup);
[ [ 6 .. 11 ], [ 5, 7, 8, 9, 11 ], [ 5, 6, 8, 9 ], [ 5, 6, 7, 9 ],
[ 5, 6, 7, 8 ], [ 4, 6, 7 ], [ 4, 7, 9, 10 ], [ 4, 6, 9, 11 ],
[ 4, 5, 11 ], [ 3, 8, 10 ], [ 3, 7, 11 ], [ 2, 11 ] ]

```
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