In this chapter we explain how to compute a minimal presentation of a numerical semigroup. There are three functions involved in this process.

`‣ MinimalPresentationOfNumericalSemigroup` ( S ) | ( function ) |

`S` is a numerical semigroup. The output is a list of lists with two elements. Each list of two elements represents a relation between the minimal generators of the numerical semigroup. If { {x_1,y_1},...,{x_k,y_k}} is the output and {m_1,...,m_n} is the minimal system of generators of the numerical semigroup, then {x_i,y_i}={{a_i_1,...,a_i_n},{b_i_1,...,b_i_n}} and a_i_1m_1+⋯+a_i_nm_n= b_i_1m_1+ ⋯ +b_i_nm_n.

Any other relation among the minimal generators of the semigroup can be deduced from the ones given in the output.

The algorithm implemented is described in [Ros96a] (see also [RG99a]).

gap> s:=NumericalSemigroup(3,5,7); <Numerical semigroup with 3 generators> gap> MinimalPresentationOfNumericalSemigroup(s); [ [ [ 0, 2, 0 ], [ 1, 0, 1 ] ], [ [ 3, 1, 0 ], [ 0, 0, 2 ] ], [ [ 4, 0, 0 ], [ 0, 1, 1 ] ] ]

The first element in the list means that 1× 3+1× 7=2× 5, and the others have similar meanings.

`‣ GraphAssociatedToElementInNumericalSemigroup` ( n, S ) | ( function ) |

`S` is a numerical semigroup and `n` is an element in `S`.

The output is a pair. If {m_1,...,m_n} is the set of minimal generators of `S`, then the first component is the set of vertices of the graph associated to `n` in `S`, that is, the set { m_i | n-m_i∈ S}, and the second component is the set of edges of this graph, that is, { {m_i,m_j} | n-(m_i+m_j)∈ S}.

This function is used to compute a minimal presentation of the numerical semigroup `S`, as explained in [Ros96a].

gap> s:=NumericalSemigroup(3,5,7);; gap> GraphAssociatedToElementInNumericalSemigroup(10,s); [ [ 3, 5, 7 ], [ [ 3, 7 ] ] ]

`‣ BettiElementsOfNumericalSemigroup` ( S ) | ( function ) |

`S` is a numerical semigroup.

The output is the set of elements in `S` whose associated graph is nonconnected [GO10].

gap> s:=NumericalSemigroup(3,5,7);; gap> BettiElementsOfNumericalSemigroup(s); [ 10, 12, 14 ]

`‣ PrimitiveElementsOfNumericalSemigroup` ( S ) | ( function ) |

`S` is a numerical semigroup.

The output is the set of elements s in `S` such that there exists a minimal solution to msg⋅ x-msg⋅ y = 0, such that x,y are factorizations of s, and msg is the minimal generating system of `S`. Betti elements are primitive, but not the way around in general.

gap> s:=NumericalSemigroup(3,5,7);; gap> PrimitiveElementsOfNumericalSemigroup(s); [ 3, 5, 7, 10, 12, 14, 15, 21, 28, 35 ]

`‣ ShadedSetOfElementInNumericalSemigroup` ( n, S ) | ( function ) |

`S` is a numerical semigroup and `n` is an element in `S`.

The output is a simplicial complex C. If {m_1,...,m_n} is the set of minimal generators of `S`, then L ∈ C if n-∑_i∈ L m_i∈ S ([SW86]).

This function is a generalization of the graph associated to `n`.

gap> s:=NumericalSemigroup(3,5,7);; gap> ShadedSetOfElementInNumericalSemigroup(10,s); [ [ ], [ 3 ], [ 3, 7 ], [ 5 ], [ 7 ] ]

A numerical semigroup S is uniquely presented if for any two minimal presentations σ and τ and any (a,b)∈ σ, either (a,b)∈ τ or (b,a)∈ τ, that is, there is essentially a unique minimal presentation (up to arrangement of the components of the pairs in it).

`‣ IsUniquelyPresentedNumericalSemigroup` ( S ) | ( function ) |

`S` is a numerical semigroup.

The output is true if `S` has uniquely presented. The implementation is based on (see [GO10]).

gap> s:=NumericalSemigroup(3,5,7);; gap> IsUniquelyPresentedNumericalSemigroup(s); true

`‣ IsGenericNumericalSemigroup` ( S ) | ( function ) |

`S` is a numerical semigroup.

The output is true if `S` has a generic presentation, that is, in every minimal relation all generators occur. These semigroups are uniquely presented (see [BGG11]).

gap> s:=NumericalSemigroup(3,5,7);; gap> IsGenericNumericalSemigroup(s); true

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