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<Chapter Label = "Standard examples">
  <Heading>
    Standard examples
  </Heading>

  In this chapter we describe some standard families of examples of semigroups
  and monoids which are available in the &SEMIGROUPS; package.
  <P/>

  
  

  <Section Label = "Transformation semigroups">
    <Heading>
      Transformation semigroups
    </Heading>

    In this section, we describe the operations in &SEMIGROUPS; that can be
    used to create transformation semigroups belonging to several standard
    classes of example. See <Ref Chap = "Transformations" BookName = "ref" />
    for more information about transformations.
    <#Include Label = "CatalanMonoid">
    <#Include Label = "EndomorphismsPartition">
    <#Include Label = "PartialTransformationMonoid">
    <#Include Label = "SingularTransformationSemigroup">
    <#Include Label = "OrderPreserving">
    <#Include Label = "EndomorphismMonoid">

  </Section>

  
  

  <Section Label = "Semigroups of partial permutations">
    <Heading>
      Semigroups of partial permutations
    </Heading>

    In this section, we describe the operations in &SEMIGROUPS; that can be used
    to create semigroups of partial permutations belonging to several standard
    classes of example. See <Ref Chap = "Partial permutations" BookName = "ref"
      /> for more information about partial permutations.
    <#Include Label = "MunnSemigroup">
    <#Include Label = "RookMonoid">
    <#Include Label = "POIPODIPOPIPORI">
  </Section>

  
  

  <Section Label = "Semigroups of bipartitions">
    <Heading>
      Semigroups of bipartitions
    </Heading>

    In this section, we describe the operations in &SEMIGROUPS; that can be
    used to create bipartition semigroups belonging to several standard
    classes of example. See Chapter <Ref Chap = "Bipartitions and blocks"/> for
    more information about bipartitions.
    <#Include Label = "PartitionMonoid">
    <#Include Label = "BrauerMonoid">
    <#Include Label = "JonesMonoid">
    <#Include Label = "PartialJonesMonoid">
    <#Include Label = "AnnularJonesMonoid">
    <#Include Label = "MotzkinMonoid">
    <#Include Label = "DualSymmetricInverseSemigroup">
    <#Include Label = "UniformBlockBijectionMonoid">
    <#Include Label = "PlanarPartitionMonoid">
    <#Include Label = "ModularPartitionMonoid">
    <#Include Label = "ApsisMonoid">
  </Section>

  
  

  <Section Label = "Standard PBR semigroups">
    <Heading>
      Standard PBR semigroups
    </Heading>

    In this section, we describe the operations in &SEMIGROUPS; that can be used
    to create standard examples of semigroups of partitioned binary relations
    (PBRs).  See Chapter <Ref Chap = "Partitioned binary relations (PBRs)"/> for
    more information about PBRs.
    <#Include Label = "FullPBRMonoid">
  </Section>

  
  

  <Section Label = "Semigroups of matrices of a finite field">
    <Heading>
      Semigroups of matrices over a finite field
    </Heading>
    In this section, we describe the operations in &SEMIGROUPS; that can be used
    to create semigroups of matrices over a finite field that belonging to
    several standard classes of example. See the section
    <Ref Sect = "Matrices over finite fields" Style="Text" /> for more
    information about matrices over a finite field.
    <#Include Label = "GeneralLinearMonoid">
    <#Include Label = "SpecialLinearMonoid">
    
    <#Include Label = "IsFullMatrixMonoid">
  </Section>

  
  

  <Section Label = "Semigroups of boolean matrices">
    <Heading>
      Semigroups of boolean matrices
    </Heading>
    In this section, we describe the operations in &SEMIGROUPS; that can be used
    to create semigroups of boolean matrices belonging to several standard
    classes of example. See the section <Ref Sect = "Boolean matrices"
      Style="Text" /> for more information about boolean matrices.
    <#Include Label = "FullBooleanMatMonoid">
    <#Include Label = "RegularBooleanMatMonoid">
    <#Include Label = "ReflexiveBooleanMatMonoid">
    <#Include Label = "HallMonoid">
    <#Include Label = "GossipMonoid">
    <#Include Label = "TriangularBooleanMatMonoid">
  </Section>

  
  

  <Section Label = "Semigroups of matrices over a semiring">
    <Heading>
      Semigroups of matrices over a semiring
    </Heading>
    In this section, we describe the operations in &SEMIGROUPS; that can be used
    to create semigroups of matices over a semiring that belong to several
    standard classes of example.  See Chapter
    <Ref Chap = "Matrices over semirings" /> for more information about matrices
    over a semiring.
    <#Include Label = "FullTropicalMaxPlusMonoid"/>
    <#Include Label = "FullTropicalMinPlusMonoid"/>
  </Section>

  
  

  <Section Label = "Examples in various representations">
    <Heading>
      Examples in various representations
    </Heading>

    In this section, we describe the functions in &SEMIGROUPS; that can be used
    to create standard semigroups in various representations.  For all of these
    examples, the default representation is as a semigroup of transformations.
    In general, these functions do not return a representation of minimal
    degree.

    <#Include Label = "TrivialSemigroup">
    <#Include Label = "MonogenicSemigroup">
    <#Include Label = "RectangularBand">
    <#Include Label = "FreeSemilattice">
    <#Include Label = "ZeroSemigroup">
    <#Include Label = "LeftZeroSemigroup">
    <#Include Label = "BrandtSemigroup">
  </Section>

  
  

  <Section Label="Free bands">
    <Heading>
      Free bands
    </Heading>

    This chapter describes the functions in &SEMIGROUPS; for dealing with free
    bands. This part of the manual and the functions described herein were
    originally written by Julius Jonušas, with later additions by Reinis
    Cirpons, Tom Conti-Leslie, and Murray Whyte<P/>

    A semigroup <M>B</M> is a <E>free band</E> on a non-empty set <M>X</M> if
    <M>B</M> is a band with a map <M> f </M> from <M> B </M> to <M>X</M> such that
    for every band <M> S </M> and every map <M> g </M> from <M>X</M> to <M> B </M>
    there exists a unique homomorphism <M> g' from B to S such
    that <M>fg' = g. The free band on a set X is unique up to
    isomorphism.  Moreover, by the universal property, every band can be expressed
    as a quotient of a free band.<P/>

    For an alternative description of a free band. Suppose that <M> X </M> is a
    non-empty set and <M> X ^ + </M> a free semigroup on <M> X </M>. Also suppose
    that <M> b </M> is the smallest congurance on <M> X ^ + </M> containing the set
    <Display> \{(w ^ 2, w) : w \in X ^ + \}. </Display> Then the free band on
    <M> X </M> is isomorphic to the quotient of <M> X ^ + </M> by <M> b </M>.
    See Section 4.5 of <Cite Key = "Howie1995aa" /> for more information on
    free bands.

    <#Include Label = "FreeBand">
    <#Include Label = "IsFreeBandCategory">
    <#Include Label = "IsFreeBand">
    <#Include Label = "IsFreeBandElement">
    <#Include Label = "IsFreeBandElementCollection">
    <#Include Label = "IsFreeBandSubsemigroup">
    <#Include Label = "ContentOfFreeBandElement">
    <#Include Label = "EqualInFreeBand">
    <#Include Label = "GreensDClassOfElement" >

    <Subsection>
      <Heading>Operators</Heading>
      The following operators are also included for free band elements:
      <List>
        <Mark><C><A>u</A> * <A>v</A></C></Mark>
        <Item>
          returns the product of two free band elements <A>u</A> and
          <A>v</A>.
        </Item>
        <Mark><C><A>u</A> = <A>v</A> </C></Mark>
        <Item>
          checks if two free band elements are equal.
        </Item>
        <Mark><C><A>u</A> < <A>v</A> </C></Mark>
        <Item>
          compares the sizes of the internal representations of two free
          band elements.
        </Item>
      </List>
    </Subsection>
  </Section>

  
  

  <Section Label="Graph inverse semigroups">
    <Heading>
      Graph inverse semigroups
    </Heading>

    In this chapter we describe a class of semigroups arising from directed
    graphs. <P/>

    The functionality in &SEMIGROUPS; for graph inverse semigroups was written
    jointly by Zak Mesyan (UCCS) and J. D. Mitchell (St Andrews). The
    functionality for graph inverse semigroup congruences was written by Marina
    Anagnostopoulou-Merkouri (St Andrews).
    <P/>

    <#Include Label = "GraphInverseSemigroup">
    <#Include Label = "RangeSourceGraphInverseSemigroupElement">
    <#Include Label = "IsVertex">
    <#Include Label = "IsGraphInverseSemigroup">
    <#Include Label = "GraphOfGraphInverseSemigroup">
    <#Include Label = "IsGraphInverseSemigroupElementCollection">
    <#Include Label = "IsGraphInverseSubsemigroup">
    <#Include Label = "VerticesOfGraphInverseSemigroup">
    <#Include Label = "IndexOfVertexOfGraphInverseSemigroup">

  </Section>

  
  

  <Section Label="Free inverse semigroups">
    <Heading>
      Free inverse semigroups
    </Heading>

    This chapter describes the functions in &SEMIGROUPS; for dealing with free
    inverse semigroups. This part of the manual and the functions described
    herein were written by Julius Jonušas.<P/>

    An inverse semigroup <M>F</M> is said to be <E>free</E> on a non-empty set
    <M>X</M> if there is a map <M>f</M> from <M>F</M> to <M>X</M> such that for
    every inverse semigroup <M>S</M> and a map <M>g</M> from <M>X</M> to
    <M>S</M> there exists a unique homomorphism <M>g' from F to
    <M>S</M> such that <M>fg' = g. Moreover, by this universal property,
    every inverse semigroup can be expressed as a quotient of a free inverse
    semigroup.
    <P/>

    The internal representation of an element of a free inverse semigroup
    uses a Munn tree. A <E>Munn tree</E> is a directed tree with distinguished
    start and terminal vertices and where the edges are labeled by generators so
    that two edges labeled by the same generator are only incident to the same
    vertex if one of the edges is coming in and the other is leaving the vertex.
    For more information regarding free inverse semigroups and the Munn
    representations see Section 5.10 of <Cite Key = "Howie1995aa"/>.
    <P/>

    See also <Ref Chap = "Inverse semigroups and monoids" BookName = "ref"/>,
    <Ref Chap = "Partial permutations" BookName = "ref"/> and <Ref Sect = "Free
      Groups, Monoids and Semigroups" BookName = "ref" />.
    <P/>

    An element of a free inverse semigroup in &SEMIGROUPS; is displayed, by
    default, as a shortest word corresponding to the element.  However, there
    might be more than one word of the minimum length. For example, if <M>x</M>
    and <M>y</M> are generators of a free inverse semigroups, then <Display>xyy
      ^ {-1}xx ^ {-1}x ^ {-1} = xxx ^ {-1}yy ^ {-1}x ^ {-1}.</Display> See <Ref
      Attr = "MinimalWord" Label = "for free inverse semigroup element"/>.
    Therefore we provide a another method for printing elements of a free
    inverse semigroup: a unique canonical form.  Suppose an element of a free
    inverse semigroup is given as a Munn tree. Let <M>L</M> be the set of words
    corresponding to the shortest paths from the start vertex to the leaves of
    the tree. Also let <M>w</M> be the word corresponding to the shortest path
    from the start vertex to the terminal vertex. The word <M>vv ^ {-1}</M> is an
    idempotent for every <M>v</M> in <M>L</M>.
    The canonical form is given by multiplying
    these idempotents, in shortlex order, and then postmultiplying by <M>w</M>.
    For example, consider the word <M>xyy ^ {-1}xx ^ {-1}x ^ {-1}</M> again.
    The words corresponding to the paths to the leaves are in this case
    <M>xx</M> and <M>xy</M>. And <M>w</M> is an empty word since start and
    terminal vertices are the same. Therefore, the canonical form is
    <Display>xxx ^ {-1}x ^ {-1}xyy ^ {-1}x ^ {-1}.</Display> See <Ref Oper =
      "CanonicalForm" Label = "for a free inverse semigroup element"/>.

    <#Include Label = "FreeInverseSemigroup">
    <#Include Label = "IsFreeInverseSemigroupCategory">
    <#Include Label = "IsFreeInverseSemigroup">
    <#Include Label = "IsFreeInverseSemigroupElement">
    <#Include Label = "IsFreeInverseSemigroupElementCollection">
    <#Include Label = "CanonicalForm">
    <#Include Label = "MinimalWord">
    <Subsection>
      <Heading>Displaying free inverse semigroup elements </Heading>
    <Heading>Displaying free inverse semigroup elements</Heading>
    There is a way to change how &GAP; displays free inverse semigroup
    elements using the user preference <C>FreeInverseSemigroupElementDisplay</C>.
    See <Ref Func = "UserPreference" BookName = "ref"/> for more information
    about user preferences.<P/>

    There are two possible values for <C>FreeInverseSemigroupElementDisplay</C>:
    <List>
      <Mark>minimal </Mark>
      <Item> With this option selected, &GAP; will display a shortest word
        corresponding to the free inverse semigroup element. However,
        this shortest word is not unique. This is a default setting.
      </Item>

      <Mark>canonical</Mark>
      <Item> With this option selected, &GAP; will display a free inverse
        semigroup element in the canonical form.
      </Item>
    </List>

    <Example><![CDATA[
gap> SetUserPreference("semigroups",
>                      "FreeInverseSemigroupElementDisplay",
>                      "minimal");
gap> S := FreeInverseSemigroup(2);
<free inverse semigroup on the generators [ x1, x2 ]>
gap> S.1 * S.2;
x1*x2
gap> SetUserPreference("semigroups",
>                      "FreeInverseSemigroupElementDisplay",
>                      "canonical");
gap> S.1 * S.2;
x1x2x2^-1x1^-1x1x2]]></Example>
    </Subsection>

    <Subsection
     Label="Operators and operations for free inverse semigroup elements">
      <Heading>Operators for free inverse semigroup elements
      </Heading>
      <List>
        <Mark><C><A>w</A> ^ -1</C></Mark>
        <Item>
          returns the semigroup inverse of the free inverse semigroup element
          <A>w</A>.
        </Item>

        <Mark><C><A>u</A> * <A>v</A></C></Mark>
        <Item>
          returns the product of two free inverse semigroup elements <A>u</A>
          and <A>v</A>.
        </Item>
        <Mark><C><A>u</A> = <A>v</A> </C></Mark>
        <Item>
          checks if two free inverse semigroup elements are equal, by comparing
          their canonical forms.
        </Item>
      </List>

    </Subsection>
  </Section>

</Chapter>

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