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

  In this chapter we describe some standard ways of constructing semigroups and
  monoids from other semigroups that are available in the &SEMIGROUPS; package.
  <P/>

  <Section Label="Products of semigroups">
    <Heading>
     Products of semigroups
    </Heading>
    In this section, we describe the functions in &SEMIGROUPS; that can be used
    to create various products of semigroups.

    <#Include Label = "DirectProduct">
    <#Include Label = "WreathProduct">
  </Section>

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  <Section>
    <Heading> Dual semigroups </Heading>
    The <E>dual semigroup</E> of a semigroup <C>S</C> is the semigroup with the
    same underlying set of elements but with reversed multiplication; this is
    anti-isomorphic to <C>S</C>. In &SEMIGROUPS; a semigroup and its dual
    are represented with disjoint sets of elements.
    <#Include Label = "DualSemigroup">
    <#Include Label = "IsDualSemigroupRep">
    <#Include Label = "IsDualSemigroupElement">
    <#Include Label = "AntiIsomorphismDualSemigroup">
  </Section>

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  <Section Label = "Strong semilattices of semigroups">
    <Heading>
      Strong semilattices of semigroups
    </Heading>

    In this section, we describe how &SEMIGROUPS; can be used to create and
    manipulate strong semilattices of semigroups (SSSs).
    Strong semilattices of semigroups are described, for example, in Section
    4.1 of <Cite Key = "Howie1995aa"/>.
    They consist of a meet-semilattice <M>Y</M> along with a collection of
    semigroups <M>S_a</M> for each <M>a</M> in <M>Y</M>, and a collection of
    homomorphisms <M>f_{ab} : S_a \rightarrow S_b</M> for each <M>a</M> and
    <M>b</M> in <M>Y</M> such that <M>a \geq b</M>.
    <P/>

    The product of two elements <M>x \in S_a, y \in S_b</M> is defined to lie
    in the semigroup <M>S_c</M>, corresponding to the meet <M>c</M> of
    <M>a, b \in Y</M>. The exact element of <M>S_c</M> equal to the product
    is obtained using the homomorphisms of the SSS: <M>xy = (x f_{ac})
    (y f_{bc})</M>.

    <#Include Label = "StrongSemilatticeOfSemigroups">
    <#Include Label = "SSSE">
    <#Include Label = "IsSSSE">
    <#Include Label = "IsStrongSemilatticeOfSemigroups">
    <#Include Label = "SemilatticeOfStrongSemilatticeOfSemigroups">
    <#Include Label = "SemigroupsOfStrongSemilatticeOfSemigroups">
    <#Include Label = "HomomorphismsOfStrongSemilatticeOfSemigroups">
  </Section>

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  <Section Label = "McAlister triple semigroups">
    <Heading>
      McAlister triple semigroups
    </Heading>
    In this section, we describe the functions in &GAP; for creating and
    computing with McAlister triple semigroups and their subsemigroups.
    This implementation is based on the section in Chapter 5 of
    <Cite Key = "Howie1995aa"/> but differs from the treatment in Howie
    by using right actions instead of left. Some definitions found in
    the documentation are changed for this reason. <P/>

    The importance of the McAlister triple semigroups lies in the fact that
    they are exactly the E-unitary inverse semigroups, which are an important
    class in the study of inverse semigroups. <P/>

    First we define E-unitary inverse semigroups. It is standard to
    denote the subsemigroup of a semigroup consisting of its idempotents by
    <C>E</C>. A semigroup <C>S</C> is said to be <E>E-unitary</E> if for all
    <C>e</C> in <C>E</C> and for all <C>s</C> in <C>S</C>:

    <List>
      <Item>
        <C>es</C> <M>\in</M> <C>E</C> implies <C>s</C> <M>\in</M> <C>E</C>,
      </Item>
      <Item>
        <C>se</C> <M>\in</M> <C>E</C> implies <C>s</C> <M>\in</M> <C>E</C>.
      </Item>
    </List>

    For inverse semigroups these two conditions are equivalent. We are only
    interested in <E>E-unitary inverse semigroups</E>.
    Before defining McAlister triple semigroups we define a McAlister triple.
    A <E>McAlister triple</E> is a triple <C>(G,X,Y)</C> which consists of:

    <List>
      <Item>
          a partial order <C>X</C>,
        </Item>
        <Item>
            a subset <C>Y</C> of <C>X</C>,
          </Item>
          <Item>
            a group <C>G</C> which acts on <C>X</C>, on the right, by order
            automorphisms. That means for all <C>A,B</C> <M>\in</M> <C>X</C> and
            for all <C>g</C> <M>\in</M> <C>G</C>: <C>A</C> <M>\leq</M> <C>B</C> if
              and only if <C>Ag</C> <M>\leq</M> <C>Bg</C>.
            </Item>
          </List>

          Furthermore, <C>(G,X,Y)</C> must satisfy the following four properties to
          be a McAlister triple:

          <List>
            <Mark> M1 </Mark>
            <Item>
              <C>Y</C> is a subset of <C>X</C> which is a join-semilattice together
                with the restriction of the order relation of <C>X</C> to <C>Y</C>.
              </Item>
              <Mark> M2 </Mark>
              <Item>
                <C>Y</C> is an order ideal of <C>X</C>. That is to say, for all <C>A</C>
                <M>\in</M> <C>X</C> and for all <C>B</C> <M>\in</M>
                <C>Y</C>: if <C>A</C> <M>\leq</M> <C>B</C>, then <C>A</C> <M>\in</M>
                <C>Y</C>.
              </Item>
              <Mark> M3 </Mark>
              <Item>
                Every element of <C>X</C> is the image of some element in <C>Y</C> moved
                  by an element of <C>G</C>. That is to say, for every <C>A</C> <M>\in</M>
                  <C>X</C>, there exists some <C>B</C> <M>\in</M> <C>Y</C> and there exists
                  <C>g</C> <M>\in</M> <C>G</C> such that <C>A</C> = <C>Bg</C>.
                </Item>
                <Mark> M4 </Mark>
                <Item>
                  Finally, for all <C>g</C> <M>\in</M> <C>G</C>, the intersection
                  <C>{yg : y </C><M>\in</M><C> Y}</C> <M>\cap</M> <C>Y</C> is non-empty.
                </Item>
              </List>

              We may define an E-unitary inverse semigroup using a McAlister triple. Given
              <C>(G,X,Y)</C> let <C>M(G,X,Y)</C> be the set of all pairs <C>(A,g)</C> in
              <C>Y x G</C> such that <C>A</C> acted on by the inverse of <C>g</C> is in
              <C>Y</C> together with multiplication defined by <P/>

              <C>(A,g)*(B,h) = (Join(A,Bg^-1),hg)</C> <P/>

              where <C>Join</C> is the natural join operation of the semilattice and
              <C>Bg^-1</C> is <C>B</C> acted on by the inverse of <C>g</C>. With this
              operation, <C>M(G,X,Y)</C> is a semigroup which we call a <E>McAlister
              triple semigroup</E> over <C>(G,X,Y)</C>. In fact every McAlister triple
              semigroup is an E-unitary inverse semigroup and every E-unitary inverse
              semigroup is isomorphic to some McAlister triple semigroup. Note that there
              need not be a unique McAlister triple semigroup for a particular McAlister
              triple because in general there is more than one way for a group to act on
              a partial order.

    <#Include Label = "IsMcAlisterTripleSemigroup">
    <#Include Label = "McAlisterTripleSemigroup">
    <#Include Label = "McAlisterTripleSemigroupGroup">
    <#Include Label = "McAlisterTripleSemigroupPartialOrder">
    <#Include Label = "McAlisterTripleSemigroupSemilattice">
    <#Include Label = "McAlisterTripleSemigroupAction">
    <#Include Label = "IsMcAlisterTripleSemigroupElement">
    <#Include Label = "McAlisterTripleSemigroupElement">
  </Section>


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</Chapter>
