chap1.txt

  
  1 Introduction
  
  A  numerical semigroup is a subset of the set N of nonnegative integers that
  is  closed  under  addition, contains 0 and whose complement in N is finite.
  The  smallest  positive  integer  belonging  to a numerical semigroup is its
  multiplicity.
  
  Let  S  be  a numerical semigroup and A be a subset of S. We say that A is a
  system  of  generators  of  S  if S={ k_1 a_1+⋯+ k_n a_n | n,k_1,...,k_n∈ N,
  a_1,...,a_n∈  A}.  The  set  A  is a minimal system of generators of S if no
  proper subset of A is a system of generators of S.
  
  Every numerical semigroup has a unique minimal system of generators. This is
  a  data  that can be used in order to uniquely define a numerical semigroup.
  Observe  that  since  the  complement of a numerical semigroup in the set of
  nonnegative  integers  is  finite,  this  implies  that  the greatest common
  divisor  of  the  elements  of  a  numerical  semigroup  is  1, and the same
  condition  must  be fulfilled by its minimal system of generators (or by any
  of its systems of generators).
  
  Given  a  numerical  semigroup  S  and  a  nonzero  element s in it, one can
  consider  for every integer i ranging from 0 to s-1, the smallest element in
  S  congruent  with  i  modulo  s,  say  w(i)  (this element exists since the
  complement  of  S  in  N  is  finite).  Clearly  w(0)=0.  The  set Ap(S,s)={
  w(0),w(1),...,  w(s-1)} is called the Apéry set of S with respect to s. Note
  that  a  nonnegative integer x congruent with i modulo s belongs to S if and
  only  if  w(i)≤ x. Thus the pair (s, Ap(S,s)) fully determines the numerical
  semigroup  S  (and can be used to easily solve the membership problem to S).
  This  set  is  in  fact  one of the most powerfull tools known for numerical
  semigroups,  and  it  is  used  almost  everywhere  in  the  computation  of
  components  and  invariants associated to a numerical semigroup. Usually the
  element  s  is taken to be the multiplicity, since in this way the resulting
  Apéry set is the smallest possible.
  
  A  gap  of a numerical semigroup S is a nonnegative integer not belonging to
  S.  The  set of gaps of S is usually denoted by H(S), and clearly determines
  uniquely  S.  Note  that if x is a gap of S, then so are all the nonnegative
  integers dividing it. Thus in order to describe S we do not need to know all
  its  gaps, but only those that are maximal with respect to the partial order
  induced by division in N. These gaps are called fundamental gaps.
  
  The  largest nonnegative integer not belonging to a numerical semigroup S is
  the  Frobenius  number  of  S. If S is the set of nonnegative integers, then
  clearly its Frobenius number is -1, otherwise its Frobenius number coincides
  with  the  maximum  of  the  gaps  (or fundamental gaps) of S. The Frobenius
  number  plus one is known as the conductor of the semigroup. In this package
  we refer to the elements in the semigroup that are less than or equal to the
  conductor  as  small  elements  of  the  semigroup.  Observe  that  from the
  definition,  if  S is a numerical semigroup with Frobenius number f, then f+
  N∖{0}⊆ S. An integer z is a pseudo-Frobenius number of S if z+S∖{0}⊆ S. Thus
  the  Frobenius  number of S is one of its pseudo-Frobenius numbers. The type
  of   a   numerical   semigroup   is  the  cardinality  of  the  set  of  its
  pseudo-Frobenius numbers.
  
  The  number  of  numerical  semigroups  having  a  given Frobenius number is
  finite.  The  elements  in this set of numerical semigroups that are maximal
  with  respect to set inclusion are precisely those numerical semigroups that
  cannot  be  expressed  as  intersection  of  two  other numerical semigroups
  containing  them  properly, and thus they are known as irreducible numerical
  semigroups.  Clearly,  every  numerical  semigroup  is  the  intersection of
  (finitely many) irreducible numerical semigroups.
  
  A  numerical  semigroup  S with Frobenius number f is symmetric if for every
  integer  x,  either  x∈  S  or  f-x∈  S.  The  set  of irreducible numerical
  semigroups  with  odd  Frobenius  number coincides with the set of symmetric
  numerical  semigroups. The numerical semigroup S is pseudo-symmetric if f is
  even and for every integer x not equal to f/2 either x∈ S or f-x∈ S. The set
  of  irreducible numerical semigroups with even Frobenius number is precisely
  the  set  of  pseudo-symmetric  numerical  semigroups.  These two classes of
  numerical semigroups have been widely studied in the literature due to their
  nice  applications  in  Algebraic Geometry. This is probably one of the main
  reasons  that made people turn their attention on numerical semigroups again
  in   the   last   decades.   Symmetric  numerical  semigroups  can  be  also
  characterized  as  those  with  type  one,  and  pseudo-symmetric  numerical
  semigroups  are  those  numerical semigroups with type two and such that its
  pseudo-Frobenius  numbers  are its Frobenius number and its Frobenius number
  divided by two.
  
  Another  class  of  numerical  semigroups  that  catched  the  attention  of
  researchers working on Algebraic Geometry and Commutative Ring Theory is the
  class   of  numerical  semigroups  with  maximal  embedding  dimension.  The
  embedding  dimension  of  a  numerical  semigroup  is the cardinality of its
  minimal  system  of generators. It can be shown that the embedding dimension
  is  at  most  the  multiplicity  of  the  numerical  semigroup. Thus maximal
  embedding  dimension numerical semigroups are those numerical semigroups for
  which  their  embedding dimension and multiplicity coincide. These numerical
  semigroups  have  nice  maximal  properties, not only (of course) related to
  their  embedding  dimension, but also by means of their presentations. Among
  maximal  embedding  dimension  there are two classes of numerical semigroups
  that  have  been  studied  due  to  the  connections with the equivalence of
  algebroid  branches. A numerical semigroup S is Arf if for every x≥ y≥ z∈ S,
  then  x+y-z∈  S;  and  it  is saturated if the following condition holds: if
  s,s_1,...,s_r∈  S are such that s_i≤ s for all i∈ {1,...,r} and z_1,...,z_r∈
  Z are such that z_1s_1+⋯+z_rs_r≥ 0, then s+z_1s_1+⋯ +z_rs_r∈ S.
  
  If  we  look  carefully  inside  the  set of fundamental gaps of a numerical
  semigroup,  we see that there are some fulfilling the condition that if they
  are  added to the given numerical semigroup, then the resulting set is again
  a  numerical  semigroup.  These  elements  are  called  special  gaps of the
  numerical semigroup. A numerical semigroup other than the set of nonnegative
  integers is irreducible if and only if it has only a special gap.
  
  The inverse operation to the one described in the above paragraph is that of
  removing  an  element of a numerical semigroup. If we want the resulting set
  to  be a numerical semigroup, then the only thing we can remove is a minimal
  generator.
  
  Let  a,b,c,d  be positive integers such that a/b < c/d, and let I=[a/b,c/d].
  Then  the  set  S(I)=  N∩ ⋃_n≥ 0 n I is a numerical semigroup. This class of
  numerical  semigroups  coincides with that of sets of solutions to equations
  of  the  form  A  x  mod  B  ≤ C x with A,B,C positive integers. A numerical
  semigroup in this class is said to be proportionally modular.
  
  A  sequence  of positive rational numbers a_1/b_1 < ⋯ < a_n/b_n with a_i,b_i
  positive  integers is a Bézout sequence if a_i+1b_i - a_i b_i+1=1 for all i∈
  {1,...,n-1}.  If  a/b=a_1/b_1  <  ⋯  <  a_n/b_n  =c/d,  then  S([a/b,c/d])=⟨
  a_1,...,a_n⟩.  Bézout sequences are not only interesting for this fact, they
  have  shown  to  be  a  major  tool  in  the study of proportionally modular
  numerical semigroups.
  
  If  S  is  a  numerical  semigroup and k is a positive integer, then the set
  S/k={ x∈ N | kx∈ S} is a numerical semigroup, known as the quotient S by k.
  
  Let  m  be a positive integer. A subadditive function with period m is a map
  f:  N->  N  such  that  f(0)=0, f(x+y)≤ f(x)+f(y) and f(x+m)=f(x). If f is a
  subadditive  function with period m, then the set M_f={ x∈ N | f(x)≤ x} is a
  numerical  semigroup.  Moreover,  every numerical semigroup is of this form.
  Thus  a  numerical  semigroup  can be given by a subadditive function with a
  given  period. If S is a numerical semigroup and s∈ S, snot=0, and Ap(S,s)={
  w(0),w(1),...,  w(s-1)}, then f(x)=w(x mod s) is a subadditive function with
  period s such that M_f=S.
  
  Let  S  be  a  numerical  semigroup  generated by {n_1,...,n_k}. Then we can
  define  the following morphism (called sometimes the factorization morphism)
  by  φ:  N^k  ->  S,  φ(a_1,...,a_k)=a_1n_1+⋯+a_kn_k.  If  σ  is  the  kernel
  congruence of φ (that is, aσ b if φ(a)=φ(b)), then S is isomorphic to N^k/σ.
  A  presentation  for  S is a system of generators (as a congruence) of σ. If
  {n_1,...,n_p} is a minimal system of generators, then a minimal presentation
  is  a  presentation  such that none of its proper subsets is a presentation.
  Minimal  presentations  of  numerical semigroups coincide with presentations
  with  minimal  cardinality, though in general these two concepts are not the
  same for an arbitrary commutative semigroup.
  
  A set I of integers is an ideal relative to a numerical semigroup S provided
  that  I+S⊆ I and that there exists d∈ S such that d+I⊆ S. If I⊆ S, we simply
  say that I is an ideal of S. If I and J are relative ideals of S, then so is
  I-J={z∈  Z  |  z+J⊆  I},  and  it is tightly related to the operation ":" of
  ideals in a commutative ring.
  
  In  this  package  we have implemented the functions needed to deal with the
  elements exposed in this introduction.
  
  Many of the algorithms, and the necessary background to understand them, can
  be  found  in  the  monograph  [RGS09]. Some examples in this book have been
  illustrated with the help of this package. So the reader can also find there
  more examples on the usage of the functions implemented here.
  
  This package was presented in [DGSM06]. For a survey of the features of this
  package, see [DGS16].