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<Chapter Label="ch:ZxZ">
<Heading>
  Residue-Class-Wise Affine Mappings, Groups and Monoids over <M>&ZZ;^2</M>
</Heading>

<Ignore Remark="set screen width to 75, for the example tester">
<Example>
<![CDATA[
gap> SizeScreen([75,24]);;
]]>
</Example>
</Ignore>

This chapter describes how to compute with residue-class-wise affine mappings
of <M>&ZZ;^2</M> and with groups and monoids formed by them. <P/>

The rings on which we have defined residue-class-wise affine mappings so
far have all been principal ideal domains, and it has been crucial that all
nontrivial principal ideals had finite index. However, the rings
<M>&ZZ;^d</M>, <M>d > 1</M> are not principal ideal domains. Furthermore,
their principal ideals have infinite index. Therefore as moduli of
residue-class-wise affine mappings we can only use lattices of full rank,
for these are precisely the ideals of <M>&ZZ;^d</M> of finite index.
However, on the other hand we can also be more permissive and look at
<M>&ZZ;^d</M> not as a ring, but rather as a free &ZZ;-module.
The consequence of this is that then an affine mapping of <M>&ZZ;^d</M>
is not just given by <M>v \mapsto (av+b)/c</M> for some
<M>a, b, c \in &ZZ;^d</M>, but rather by <M>v \mapsto (vA+b)/c</M>,
where <M>A \in &ZZ;^{d \times d}</M>. Also for technical reasons concerning
the implementation in &GAP;, looking at <M>&ZZ;^d</M> as a free &ZZ;-module
is preferable -- in &GAP;, <C>Integers^d</C> is not a ring, and
multiplying lists of integers means forming their scalar product.



<Section Label="sec:DefinitionOfRcwaMappingsOfZxZ">
<Heading>
  The definition of residue-class-wise affine mappings of <M>&ZZ;^d</M>
</Heading>

<Index Key="rcwa mapping" Subkey="of Z x Z, definition">rcwa mapping</Index>

Let <M>d \in &NN;</M>. We call a mapping <M>f: &ZZ;^d \rightarrow &ZZ;^d</M>
<E>residue-class-wise affine</E> if there is a lattice <M>L = &ZZ;^d M</M>
where <M>M \in &ZZ;^{d \times d}</M> is a matrix of full rank, such that the
restrictions of <M>f</M> to the residue classes <M>r + L \in &ZZ;^d/L</M>
are all affine.
This means that for any residue class <M>r + L \in &ZZ;^d/L</M>,
there is a matrix <M>A_{r+L} \in &ZZ;^{d \times d}</M>, a vector
<M>b_{r+L} \in &ZZ;^d</M> and a positive integer <M>c_{r+L}</M>
such that the restriction of <M>f</M> to <M>r + L</M> is given by
<M>f|_{r + L}: \ r + L \ \rightarrow \ &ZZ;^d, \ \
v \ \mapsto \ (v \cdot A_{r+L} + b_{r+L})/c_{r+L}</M>.
For reasons of uniqueness, we assume that <M>L</M> is chosen maximal with
respect to inclusion, and that no prime factor of <M>c_{r+L}</M> divides all
coefficients of <M>A_{r+L}</M> and <M>b_{r+L}</M>. <P/>

<Index Key="rcwa mapping" Subkey="modulus">rcwa mapping</Index>
<Index Key="rcwa mapping" Subkey="class-wise translating">
  rcwa mapping
</Index>
<Index Key="class-wise translating" Subkey="definition">
  class-wise translating
</Index>
<Index Key="prime set" Subkey="definition">prime set</Index>

We call the lattice <M>L</M> the <E>modulus</E> of <M>f</M>,
written Mod(<M>f</M>).
Further we define the <E>prime set</E> of <M>f</M> as the set of
all primes which divide the determinant of at least one of the coefficients
<M>A_{r+L}</M> or which divide the determinant of <M>M</M>, and
we call the mapping <M>f</M> <E>class-wise translating</E> if all
coefficients <M>A_{r+L}</M> are identity matrices and all
coefficients <M>c_{r+L}</M> are equal to 1. <P/>

For the sake of simplicity, we identify a lattice with the Hermite normal
form of the matrix by whose rows it is spanned.

</Section>



<Section Label="sec:EnteringRcwaMappingsOfZxZ">
<Heading>
  Entering residue-class-wise affine mappings of <M>&ZZ;^2</M>
</Heading>

Residue-class-wise affine mappings of <M>&ZZ;^2</M> can be entered
using the general constructor
<Ref Meth="RcwaMapping" Label="by ring, modulus and list of coefficients"/>
or the more specialized functions
<Ref Func="ClassTransposition" Label="r1, m1, r2, m2"/>,
<Ref Func="ClassRotation" Label="cl, u"/> and
<Ref Func="ClassShift" Label="cl"/>. The arguments differ only slightly.

<ManSection>
  <Heading>
    RcwaMapping (the general constructor; methods for <M>&ZZ;^2</M>)
  </Heading>
  <Meth Name="RcwaMapping" Arg="R, L, coeffs"
        Label="by ring = Z x Z, modulus and coefficients"/>
  <Meth Name="RcwaMapping" Arg="P1, P2"
        Label="by two partitions of Z x Z into residue classes"/>
  <Meth Name="RcwaMapping" Arg="cycles"
        Label="of Z x Z, by residue class cycles"/>
  <Meth Name="RcwaMapping" Arg="f, g"
        Label="of Z x Z, by projections to coordinates"/>
  <Returns> an rcwa mapping of <M>&ZZ;^2</M>. </Returns>
  <Description>
    The above methods return
    <List>
      <Mark>(a)</Mark>
      <Item>
        the rcwa mapping of <C><A>R</A> = Integers^2</C>
        with modulus <A>L</A> and coefficients <A>coeffs</A>,
      </Item>
      <Mark>(b)</Mark>
      <Item>
        an rcwa permutation which induces a bijection between
        the partitions <A>P1</A> and <A>P2</A> of <M>&ZZ;^2</M>
        into residue classes and which is affine on the elements
        of <A>P1</A>,
      </Item>
      <Mark>(c)</Mark>
      <Item>
        an rcwa permutation with <Q>residue class cycles</Q> given
        by a list <A>cycles</A> of lists of pairwise disjoint residue classes
        of <M>&ZZ;^2</M> each of which it permutes cyclically, and
      </Item>
      <Mark>(d)</Mark>
      <Item>
        the rcwa mapping of <M>&ZZ;^2</M> whose projections to the
        coordinates are given by <A>f</A> and <A>g</A>,
      </Item>
    </List>
    respectively. <P/>

    The modulus of an rcwa mapping of <M>&ZZ;^2</M> is a lattice of full
    rank. It is represented by a matrix <A>L</A> in Hermite normal form,
    whose rows are the spanning vectors. <P/>

    A coefficient list for an rcwa mapping of <M>&ZZ;^2</M> with
    modulus <A>L</A> consists of <M>|\det(<A>L</A>)|</M> coefficient
    triples <C>[</C><M>A_{r+&ZZ;^2<A>L</A>}</M>, <M>b_{r+&ZZ;^2<A>L</A>}</M>,
    <M>c_{r+&ZZ;^2<A>L</A>}</M><C>]</C>.
    The entries <M>A_{r+&ZZ;^2<A>L</A>}</M> are <M>2 \times 2</M> integer
    matrices, the <M>b_{r+&ZZ;^2<A>L</A>}</M> are elements of <M>&ZZ;^2</M>,
    i.e. lists of two integers, and the <M>c_{r+&ZZ;^2<A>L</A>}</M> are
    integers.
    The ordering of the coefficient triples is determined by the ordering of
    the representatives of the residue classes <M>r+&ZZ;^2<A>L</A></M> in
    the sorted list returned by <C>AllResidues(Integers^2,<A>L</A>)</C>. <P/>

    The methods for the operation <C>RcwaMapping</C> perform a number of
    argument checks, which can be skipped by using <C>RcwaMappingNC</C>
    instead. <P/>

    Last but not least, regarding Method (d) it should be mentioned
    that only very special rcwa mappings of <M>&ZZ;^2</M> have
    projections to coordinates.
<Example>
<![CDATA[
gap> R := Integers^2;;
gap> twice := RcwaMapping(R,[[1,0],[0,1]],
>                           [[[[2,0],[0,2]],[0,0],1]]);       # method (a)
Rcwa mapping of Z^2: (m,n) -> (2m,2n)
gap> [4,5]^twice;
[ 8, 10 ]
gap> twice1 := RcwaMapping(R,[[1,0],[0,1]],
>                            [[[[2,0],[0,1]],[0,0],1]]);      # method (a)
Rcwa mapping of Z^2: (m,n) -> (2m,n)
gap> [4,5]^twice1;
[ 8, 5 ]
gap> Image(twice1);
(0,0)+(2,0)Z+(0,1)Z
gap> hyperbolic := RcwaMapping(R,[[1,0],[0,2]],
>                                [[[[4,0],[0,1]],[0, 0],2],
>                                 [[[4,0],[0,1]],[2,-1],2]]); # method (a)
<rcwa mapping of Z^2 with modulus (1,0)Z+(0,2)Z>
gap> IsBijective(hyperbolic);
true
gap> Display(hyperbolic);

Rcwa permutation of Z^2 with modulus (1,0)Z+(0,2)Z

            /
            | (2m,n/2)       if (m,n) in (0,0)+(1,0)Z+(0,2)Z
(m,n) |-> <  (2m+1,(n-1)/2) if (m,n) in (0,1)+(1,0)Z+(0,2)Z
            |
            \

gap> Trajectory(hyperbolic,[0,10000],20);
[ [ 0, 10000 ], [ 0, 5000 ], [ 0, 2500 ], [ 0, 1250 ], [ 0, 625 ],
  [ 1, 312 ], [ 2, 156 ], [ 4, 78 ], [ 8, 39 ], [ 17, 19 ], [ 35, 9 ],
  [ 71, 4 ], [ 142, 2 ], [ 284, 1 ], [ 569, 0 ], [ 1138, 0 ],
  [ 2276, 0 ], [ 4552, 0 ], [ 9104, 0 ], [ 18208, 0 ] ]
gap> P1 := AllResidueClassesModulo(R,[[2,1],[0,2]]);
[ (0,0)+(2,1)Z+(0,2)Z, (0,1)+(2,1)Z+(0,2)Z, (1,0)+(2,1)Z+(0,2)Z,
  (1,1)+(2,1)Z+(0,2)Z ]
gap> P2 := AllResidueClassesModulo(R,[[1,0],[0,4]]);
[ (0,0)+(1,0)Z+(0,4)Z, (0,1)+(1,0)Z+(0,4)Z, (0,2)+(1,0)Z+(0,4)Z,
  (0,3)+(1,0)Z+(0,4)Z ]
gap> g := RcwaMapping(P1,P2);                                 # method (b)
<rcwa permutation of Z^2 with modulus (2,1)Z+(0,2)Z>
gap> P1^g = P2;
true
gap> Display(g:AsTable);

Rcwa permutation of Z^2 with modulus (2,1)Z+(0,2)Z

   [m,n] mod (2,1)Z+(0,2)Z   |              Image of [m,n]
-----------------------------+-------------------------------------------
[0,0]                       | [m/2,-m+2n]
[0,1]                       | [m/2,-m+2n-1]
[1,0]                       | [(m-1)/2,-m+2n+3]
[1,1]                       | [(m-1)/2,-m+2n+2]

gap> classes := List([[[0,0],[[2,1],[0,2]]],[[1,0],[[2,1],[0,4]]],
>                     [[1,1],[[4,2],[0,4]]]],ResidueClass);
[ (0,0)+(2,1)Z+(0,2)Z, (1,0)+(2,1)Z+(0,4)Z, (1,1)+(4,2)Z+(0,4)Z ]
gap> g := RcwaMapping([classes]);                             # method (c)
<rcwa permutation of Z^2 with modulus (4,2)Z+(0,4)Z, of order 3>
gap> Permutation(g,classes);
(1,2,3)
gap> Support(g);
(0,0)+(2,1)Z+(0,2)Z U (1,0)+(2,1)Z+(0,4)Z U (1,1)+(4,2)Z+(0,4)Z
gap> Display(g);

Rcwa permutation of Z^2 with modulus (4,2)Z+(0,4)Z, of order 3

            /
            | (m+1,(-m+4n)/2)   if (m,n) in (0,0)+(2,1)Z+(0,2)Z
            | (2m-1,(m+2n+1)/2) if (m,n) in (1,0)+(2,1)Z+(0,4)Z
(m,n) |-> <  ((m-1)/2,(n-1)/2) if (m,n) in (1,1)+(4,2)Z+(0,4)Z
            | (m,n)             otherwise
            |
            \

gap> g := RcwaMapping(ClassTransposition(0,2,1,2),
>                     ClassReflection(0,2));                  # method (d)
<rcwa mapping of Z^2 with modulus (2,0)Z+(0,2)Z>
gap> Display(g);

Rcwa mapping of Z^2 with modulus (2,0)Z+(0,2)Z

            /
            | (m+1,-n) if (m,n) in (0,0)+(2,0)Z+(0,2)Z
            | (m+1,n)  if (m,n) in (0,1)+(2,0)Z+(0,2)Z
(m,n) |-> <  (m-1,-n) if (m,n) in (1,0)+(2,0)Z+(0,2)Z
            | (m-1,n)  if (m,n) in (1,1)+(2,0)Z+(0,2)Z
            |
            \

gap> g^2;
IdentityMapping( ( Integers^2 ) )
gap> List(ProjectionsToCoordinates(g),Factorization);
[ [ ( 0(2), 1(2) ) ], [ ClassReflection( 0(2) ) ] ]
]]>
</Example>
  </Description>
</ManSection>

<ManSection>
  <Heading> ClassTransposition (for <M>&ZZ;^2</M>) </Heading>
  <Func Name="ClassTransposition" Arg="r1, L1, r2, L2"
                                  Label="r1, L1, r2, L2 (for Z x Z)"/>
  <Func Name="ClassTransposition" Arg="cl1, cl2"
                                  Label="cl1, cl2 (for Z x Z)"/>
  <Returns>
    the class transposition <M>\tau_{r_1+&ZZ;^2L_1,r_2+&ZZ;^2L_2}</M>.
  </Returns>
  <Description>
    Let <M>d \in &NN;</M>, and let <M>L_1, L_2 \in &ZZ;^{d \times d}</M> be
    matrices of full rank which are in Hermite normal form. Further let
    <M>r_1 + &ZZ;^d L_1</M> and <M>r_2 + &ZZ;^d L_2</M> be disjoint residue
    classes, and assume that the representatives <M>r_1</M> and <M>r_2</M>
    are reduced modulo <M>&ZZ;^d L_1</M> and <M>&ZZ;^d L_2</M>,
    respectively. Then we define the <E>class transposition</E>
    <M>\tau_{r_1+&ZZ;^d L_1, r_2+&ZZ;^d L_2} \in {\rm Sym}(&ZZ;^d)</M> as the
    involution which interchanges <M>r_1 + k L_1</M> and <M>r_2 + k L_2</M>
    for all <M>k \in &ZZ;^d</M>. <P/>

    The class transposition <M>\tau_{r_1+&ZZ;^d L_1, r_2+&ZZ;^d L_2}</M>
    interchanges the residue classes <M>r_1+&ZZ;^d L_1</M> and
    <M>r_2+&ZZ;^d L_2</M>, and fixes the complement of their union pointwise.
    The set of all class transpositions of <M>&ZZ;^d</M> generates the
    simple group CT(<M>&ZZ;^d</M>) (cf. <Cite Key="Kohl13"/>). <P/>

    <Index Key="TransposedClasses"
           Subkey="of a class transposition of Z x Z">
      <C>TransposedClasses</C>
    </Index>

    In the four-argument form, the arguments <A>r1</A>, <A>L1</A>,
    <A>r2</A> and <A>L2</A> stand for <M>r_1</M>, <M>L_1</M>,
    <M>r_2</M> and <M>L_2</M>, respectively.
    In the two-argument form, the arguments <A>cl1</A> and <A>cl2</A>
    stand for the residue classes <M>r_1+&ZZ;^2 L_1</M>
    and <M>r_2+&ZZ;^2 L_2</M>, respectively.
    Enclosing the argument list in list brackets is permitted.
    The residue classes <M>r_1+&ZZ;^2 L_1</M> and <M>r_2+&ZZ;^2 L_2</M>
    are stored as an attribute <C>TransposedClasses</C>. <P/>

    <Index Key="SplittedClassTransposition"
           Subkey="for a class transposition of Z x Z">
      <C>SplittedClassTransposition</C>
    </Index>

    There is also a method for <C>SplittedClassTransposition</C> available
    for class transpositions of <M>&ZZ;^2</M>. This method takes as first
    argument the class transposition, and as second argument a list of
    two integers. These integers are the numbers of parts into which the
    class transposition is to be sliced in each dimension. Note that the
    product of the returned class transpositions is not always equal to
    the class transposition passed as first argument. However this equality
    holds if the first entry of the second argument is 1. <P/>
<Example>
<![CDATA[
gap> ct := ClassTransposition([0,0],[[2,1],[0,2]],[1,0],[[2,1],[0,4]]);
( (0,0)+(2,1)Z+(0,2)Z, (1,0)+(2,1)Z+(0,4)Z )
gap> Display(ct);

Rcwa permutation of Z^2 with modulus (2,1)Z+(0,4)Z, of order 2

            /
            | (m+1,(-m+4n)/2)  if (m,n) in (0,0)+(2,1)Z+(0,2)Z
(m,n) |-> <  (m-1,(m+2n-1)/4) if (m,n) in (1,0)+(2,1)Z+(0,4)Z
            | (m,n)            otherwise
            \

gap> TransposedClasses(ct);
[ (0,0)+(2,1)Z+(0,2)Z, (1,0)+(2,1)Z+(0,4)Z ]
gap> ct = ClassTransposition(last);
true
gap> SplittedClassTransposition(ct,[1,2]);
[ ( (0,0)+(2,1)Z+(0,4)Z, (1,0)+(2,1)Z+(0,8)Z ),
  ( (0,2)+(2,1)Z+(0,4)Z, (1,4)+(2,1)Z+(0,8)Z ) ]
gap> Product(last) = ct;
true
gap> SplittedClassTransposition(ct,[2,1]);
[ ( (0,0)+(4,0)Z+(0,2)Z, (1,0)+(4,2)Z+(0,4)Z ),
  ( (2,1)+(4,0)Z+(0,2)Z, (3,1)+(4,2)Z+(0,4)Z ) ]
gap> Product(last) = ct;
false
]]>
</Example>
  </Description>
</ManSection>

<ManSection>
  <Heading> ClassRotation (for <M>&ZZ;^2</M>) </Heading>
  <Func Name="ClassRotation" Arg="r, L, u" Label="r, L, u; for Z x Z"/>
  <Func Name="ClassRotation" Arg="cl, u" Label="cl, u; for Z x Z"/>
  <Returns> the class rotation <M>\rho_{r(m),u}</M>. </Returns>
  <Description>
    Let <M>d \in &NN;</M>. Given a residue class <M>r+&ZZ;^dL</M>
    and a matrix <M>u \in {\rm GL}(d,&ZZ;)</M>, the <E>class rotation</E>
    <M>\rho_{r+&ZZ;^dL,u}</M> is the rcwa mapping which maps
    <M>v \in r+&ZZ;^dL</M> to <M>vu + r(1-u)</M>
    and which fixes <M>&ZZ;^d \setminus r+&ZZ;^dL</M> pointwise.

    <Index Key="RotationFactor" Subkey="of a class rotation of Z x Z">
      <C>RotationFactor</C>
    </Index>

    In the two-argument form, the argument <A>cl</A> stands for the
    residue class <M>r+&ZZ;^dL</M>. Enclosing the argument list in list
    brackets is permitted. The argument <A>u</A> is stored as an attribute
    <C>RotationFactor</C>.
<Example>
<![CDATA[
gap> interchange := ClassRotation([0,0],[[1,0],[0,1]],[[0,1],[1,0]]);
ClassRotation( Z^2, [ [ 0, 1 ], [ 1, 0 ] ] )
gap> Display(interchange);
Rcwa permutation of Z^2: (m,n) -> (n,m)
gap> classes := AllResidueClassesModulo(Integers^2,[[2,1],[0,3]]);
[ (0,0)+(2,1)Z+(0,3)Z, (0,1)+(2,1)Z+(0,3)Z, (0,2)+(2,1)Z+(0,3)Z,
  (1,0)+(2,1)Z+(0,3)Z, (1,1)+(2,1)Z+(0,3)Z, (1,2)+(2,1)Z+(0,3)Z ]
gap> transvection := ClassRotation(classes[5],[[1,1],[0,1]]);
ClassRotation((1,1)+(2,1)Z+(0,3)Z,[[1,1],[0,1]])
gap> Display(transvection);

Tame rcwa permutation of Z^2 with modulus (2,1)Z+(0,3)Z, of order infinity

            /
            | (m,(3m+2n-3)/2) if (m,n) in (1,1)+(2,1)Z+(0,3)Z
(m,n) |-> <  (m,n)           otherwise
            |
            \
]]>
</Example>
  </Description>
</ManSection>

<ManSection>
  <Heading> ClassShift (for <M>&ZZ;^2</M>) </Heading>
  <Func Name="ClassShift" Arg="r, L, k" Label="r, L, k; for Z x Z"/>
  <Func Name="ClassShift" Arg="cl, k" Label="cl, k; for Z x Z"/>
  <Returns> the class shift <M>\nu_{r+&ZZ;^dL,k}</M>. </Returns>
  <Description>
    Let <M>d \in &NN;</M>. Given a residue class <M>r+&ZZ;^dL</M> and
    an integer <M>k \in \{1, \dots, d\}</M>, the <E>class shift</E>
    <M>\nu_{r+&ZZ;^dL,k}</M> is the rcwa mapping which maps
    <M>v \in r+&ZZ;^dL</M> to <M>v + L_k</M> and which fixes
    <M>&ZZ;^d \setminus r+&ZZ;^dL</M> pointwise.
    Here <M>L_k</M> denotes the <M>k</M>th row of <M>L</M>. <P/>

    In the two-argument form, the argument <A>cl</A> stands for the
    residue class <M>r+&ZZ;^dL</M>. Enclosing the argument list in
    list brackets is permitted.
<Example>
<![CDATA[
gap> shift1 := ClassShift([0,0],[[1,0],[0,1]],1);
ClassShift( Z^2, 1 )
gap> Display(shift1);
Tame rcwa permutation of Z^2: (m,n) -> (m+1,n)
gap> s := ClassShift(ResidueClass([1,1],[[2,1],[0,2]]),2);
ClassShift((1,1)+(2,1)Z+(0,2)Z,2)
gap> Display(s);

Tame rcwa permutation of Z^2 with modulus (2,1)Z+(0,2)Z, of order infinity

            /
            | (m,n+2) if (m,n) in (1,1)+(2,1)Z+(0,2)Z
(m,n) |-> <  (m,n)   if (m,n) in (0,0)+(2,0)Z+(0,1)Z U
            |                     (1,0)+(2,1)Z+(0,2)Z
            \
]]>
</Example>
  </Description>
</ManSection>

<Index Key="IsClassTransposition" Subkey="for an rcwa mapping of Z x Z">
  <C>IsClassTransposition</C>
</Index>
<Index Key="IsClassRotation" Subkey="for an rcwa mapping of Z x Z">
  <C>IsClassRotation</C>
</Index>
<Index Key="IsClassShift" Subkey="for an rcwa mapping of Z x Z">
  <C>IsClassShift</C>
</Index>

As for other rings, class transpositions, class rotations and
class shifts of <M>&ZZ;^2</M> have the distinguishing properties
<C>IsClassTransposition</C>, <C>IsClassRotation</C>
and <C>IsClassShift</C>.

</Section>



<Section Label="sec:MethodsForRcwaMappingsOfZxZ">
<Heading>
  Methods for residue-class-wise affine mappings of <M>&ZZ;^2</M>
</Heading>

There are methods available for rcwa mappings of <M>&ZZ;^2</M> for the
following general operations:

<Index Key="View" Subkey="for an rcwa mapping of Z x Z">
  <C>View</C>
</Index>
<Index Key="Display" Subkey="for an rcwa mapping of Z x Z">
  <C>Display</C>
</Index>
<Index Key="Print" Subkey="for an rcwa mapping of Z x Z">
  <C>Print</C>
</Index>
<Index Key="String" Subkey="for an rcwa mapping of Z x Z">
  <C>String</C>
</Index>
<Index Key="LaTeXStringRcwaMapping" Subkey="for an rcwa mapping of Z x Z">
  <C>LaTeXStringRcwaMapping</C>
</Index>
<Index Key="LaTeXAndXDVI" Subkey="for an rcwa mapping of Z x Z">
  <C>LaTeXAndXDVI</C>
</Index>
<Index Key="Modulus" Subkey="of an rcwa mapping of Z x Z">
  <C>Modulus</C>
</Index>
<Index Key="Coefficients" Subkey="of an rcwa mapping of Z x Z">
  <C>Coefficients</C>
</Index>
<Index Key="Support" Subkey="of an rcwa mapping of Z x Z">
  <C>Support</C>
</Index>
<Index Key="MovedPoints" Subkey="of an rcwa mapping of Z x Z">
  <C>MovedPoints</C>
</Index>
<Index Key="Order" Subkey="of an rcwa mapping of Z x Z">
  <C>Order</C>
</Index>
<Index Key="Multiplier" Subkey="of an rcwa mapping of Z x Z">
  <C>Multiplier</C>
</Index>
<Index Key="Divisor" Subkey="of an rcwa mapping of Z x Z">
  <C>Divisor</C>
</Index>
<Index Key="PrimeSet" Subkey="of an rcwa mapping of Z x Z">
  <C>PrimeSet</C>
</Index>
<Index Key="One" Subkey="for an rcwa mapping of Z x Z">
  <C>One</C>
</Index>
<Index Key="Zero" Subkey="for an rcwa mapping of Z x Z">
  <C>Zero</C>
</Index>
<Index Key="IsInjective" Subkey="for an rcwa mapping of Z x Z">
  <C>IsInjective</C>
</Index>
<Index Key="IsSurjective" Subkey="for an rcwa mapping of Z x Z">
  <C>IsSurjective</C>
</Index>
<Index Key="IsBijective" Subkey="for an rcwa mapping of Z x Z">
  <C>IsBijective</C>
</Index>
<Index Key="IsTame" Subkey="for an rcwa mapping of Z x Z">
  <C>IsTame</C>
</Index>
<Index Key="IsIntegral" Subkey="for an rcwa mapping of Z x Z">
  <C>IsIntegral</C>
</Index>
<Index Key="IsBalanced" Subkey="for an rcwa mapping of Z x Z">
  <C>IsBalanced</C>
</Index>
<Index Key="IsClassWiseOrderPreserving"
       Subkey="for an rcwa mapping of Z x Z">
  <C>IsClassWiseOrderPreserving</C>
</Index>
<Index Key="IsOne" Subkey="for an rcwa mapping of Z x Z">
  <C>IsOne</C>
</Index>
<Index Key="IsZero" Subkey="for an rcwa mapping of Z x Z">
  <C>IsZero</C>
</Index>
<Index Key="Trajectory" Subkey="for rcwa mappings of Z x Z">
  <C>Trajectory</C>
</Index>
<Index Key="ShortCycles"
       Subkey="for rcwa perm. of Z x Z, set of points and max. length">
  <C>ShortCycles</C>
</Index>
<Index Key="Multpk" Subkey="for rcwa mapping of Z x Z, prime and exponent">
  <C>Multpk</C>
</Index>
<Index Key="ClassWiseOrderPreservingOn" Subkey="for rcwa mappings of Z x Z">
  <C>ClassWiseOrderPreservingOn</C>
</Index>
<Index Key="ClassWiseOrderReversingOn" Subkey="for rcwa mappings of Z x Z">
  <C>ClassWiseOrderReversingOn</C>
</Index>
<Index Key="ClassWiseConstantOn" Subkey="for rcwa mappings of Z x Z">
  <C>ClassWiseConstantOn</C>
</Index>

<List>
  <Mark> Output </Mark>
  <Item>
    <C>View</C>, <C>Display</C>, <C>Print</C>, <C>String</C>,
    <C>LaTeXStringRcwaMapping</C>, <C>LaTeXAndXDVI</C>.
  </Item>
  <Mark> Access to components </Mark>
  <Item> <C>Modulus</C>, <C>Coefficients</C>. </Item>
  <Mark> Attributes </Mark>
  <Item>
    <C>Support</C> / <C>MovedPoints</C>, <C>Order</C>,
    <C>Multiplier</C>, <C>Divisor</C>, <C>PrimeSet</C>,
    <C>One</C>, <C>Zero</C>.
  </Item>
  <Mark> Properties </Mark>
  <Item>
    <C>IsInjective</C>, <C>IsSurjective</C>, <C>IsBijective</C>,
    <C>IsTame</C>, <C>IsIntegral</C>, <C>IsBalanced</C>,
    <C>IsClassWiseOrderPreserving</C>, <C>IsOne</C>, <C>IsZero</C>.
  </Item>
  <Mark> Action on <M>&ZZ;^d</M> </Mark>
  <Item>
    <C>^</C> (for points / finite sets / residue class unions),
    <C>Trajectory</C>, <C>ShortCycles</C>, <C>Multpk</C>,
    <C>ClassWiseOrderPreservingOn</C>, <C>ClassWiseOrderReversingOn</C>,
    <C>ClassWiseConstantOn</C>.
  </Item>
  <Mark> Arithmetical operations </Mark>
  <Item>
    <C>=</C>, <C>*</C> (multiplication / composition and multiplication
    by a <M>2 \times 2</M> matrix or an integer),
    <C>^</C> (exponentiation and conjugation), <C>Inverse</C>,
    <C>+</C> (addition of a constant).
  </Item>
</List>

The above operations are documented either in the &GAP; Reference Manual
or earlier in this manual. The operations which are special for rcwa mappings
of <M>&ZZ;^2</M> are described in the sequel. <P/>

<ManSection>
  <Attr Name="ProjectionsToCoordinates" Arg="f"
        Label="for an rcwa mapping of Z x Z"/>
  <Returns>
    the projections of the rcwa mapping <A>f</A> of <M>&ZZ;^2</M> to the
    coordinates if such projections exist, and <C>fail</C> otherwise.
  </Returns>
  <Description>
    An rcwa mapping can be projected to the first / second coordinate if and
    only if the first / second coordinate of the image of a point depends
    only on the first / second coordinate of the preimage. Note that this is
    a very strong and restrictive condition.
<Example>
<![CDATA[
gap> f := RcwaMapping(ClassTransposition(0,2,1,2),ClassReflection(0,2));;
gap> Display(f);

Rcwa mapping of Z^2 with modulus (2,0)Z+(0,2)Z

            /
            | (m+1,-n) if (m,n) in (0,0)+(2,0)Z+(0,2)Z
            | (m+1,n)  if (m,n) in (0,1)+(2,0)Z+(0,2)Z
(m,n) |-> <  (m-1,-n) if (m,n) in (1,0)+(2,0)Z+(0,2)Z
            | (m-1,n)  if (m,n) in (1,1)+(2,0)Z+(0,2)Z
            |
            \

gap> List(ProjectionsToCoordinates(f),Factorization);
[ [ ( 0(2), 1(2) ) ], [ ClassReflection( 0(2) ) ] ]
]]>
</Example>
  </Description>
</ManSection>

</Section>



<Section Label="sec:MethodsForRcwaGroupsOverZxZ">
<Heading>
  Methods for residue-class-wise affine groups and -monoids over <M>&ZZ;^2</M>
</Heading>

<Index Key="Rcwa"
       Subkey="the monoid formed by all rcwa permutations of Z x Z">
  <C>Rcwa</C>
</Index>
<Index Key="RCWA"
       Subkey="the group formed by all rcwa permutations of Z x Z">
  <C>RCWA</C>
</Index>
<Index Key="CT"
       Subkey="the group generated by all class transpositions of Z x Z">
  <C>CT</C>
</Index>

Residue-class-wise affine groups over <M>&ZZ;^2</M> can be entered by
<C>Group</C>, <C>GroupByGenerators</C> and <C>GroupWithGenerators</C>,
like any groups in &GAP;. Likewise, residue-class-wise affine monoids over
<M>&ZZ;^2</M> can be entered by <C>Monoid</C> and <C>MonoidByGenerators</C>.
The groups RCWA(<M>&ZZ;^2</M>) and CT(<M>&ZZ;^2</M>) are entered
as <C>RCWA(Integers^2)</C> and <C>CT(Integers^2)</C>, respectively.
The monoid Rcwa(<M>&ZZ;^2</M>) is entered as <C>Rcwa(Integers^2)</C>. <P/>

<Index Key="Size" Subkey="for an rcwa group over Z x Z">
  <C>Size</C>
</Index>
<Index Key="IsIntegral" Subkey="for an rcwa group over Z x Z">
  <C>IsIntegral</C>
</Index>
<Index Key="IsClassWiseTranslating" Subkey="for an rcwa group over Z x Z">
  <C>IsClassWiseTranslating</C>
</Index>
<Index Key="IsTame" Subkey="for an rcwa group over Z x Z">
  <C>IsTame</C>
</Index>
<Index Key="Modulus" Subkey="of an rcwa group over Z x Z">
  <C>Modulus</C>
</Index>
<Index Key="Multiplier" Subkey="of an rcwa group over Z x Z">
  <C>Multiplier</C>
</Index>
<Index Key="Divisor" Subkey="of an rcwa group over Z x Z">
  <C>Divisor</C>
</Index>

There are methods provided for the operations <C>Size</C>,
<C>IsIntegral</C>, <C>IsClassWiseTranslating</C>, <C>IsTame</C>,
<C>Modulus</C>, <C>Multiplier</C> and <C>Divisor</C>. <P/>

There are methods for <Ref Attr="IsomorphismRcwaGroup"
Label="for a group, over a given ring"/> which embed the groups SL(2,&ZZ;)
and GL(2,&ZZ;) into RCWA(<M>&ZZ;^2</M>) in such a way that the support
of the image is a specified residue class:

<ManSection>
  <Heading>
    IsomorphismRcwaGroup (Embeddings of SL(2,&ZZ;) and GL(2,&ZZ;))
  </Heading>
  <Attr Name="IsomorphismRcwaGroup"
        Arg="sl2z, cl" Label="for SL(2,Z) and a residue class"/>
  <Attr Name="IsomorphismRcwaGroup"
        Arg="gl2z, cl" Label="for GL(2,Z) and a residue class"/>
  <Returns>
    a monomorphism from <A>sl2z</A> respectively <A>gl2z</A>
    to RCWA(<M>&ZZ;^2</M>), such that the support of the image
    is the residue class <A>cl</A> and the generators
    are affine on <A>cl</A>.
  </Returns>
  <Description>
<Example>
<![CDATA[
gap> sl := SL(2,Integers);
SL(2,Integers)
gap> phi := IsomorphismRcwaGroup(sl,ResidueClass([1,0],[[2,2],[0,3]]));
[ [ [ 0, 1 ], [ -1, 0 ] ], [ [ 1, 1 ], [ 0, 1 ] ] ] ->
[ ClassRotation((1,0)+(2,2)Z+(0,3)Z,[[0,1],[-1,0]]),
  ClassRotation((1,0)+(2,2)Z+(0,3)Z,[[1,1],[0,1]]) ]
gap> Support(Image(phi));
(1,0)+(2,2)Z+(0,3)Z
gap> gl := GL(2,Integers);
GL(2,Integers)
gap> phi := IsomorphismRcwaGroup(gl,ResidueClass([1,0],[[2,2],[0,3]]));
[ [ [ 0, 1 ], [ 1, 0 ] ], [ [ -1, 0 ], [ 0, 1 ] ],
  [ [ 1, 1 ], [ 0, 1 ] ] ] ->
[ ClassRotation((1,0)+(2,2)Z+(0,3)Z,[[0,1],[1,0]]),
  ClassRotation((1,0)+(2,2)Z+(0,3)Z,[[-1,0],[0,1]]),
  ClassRotation((1,0)+(2,2)Z+(0,3)Z,[[1,1],[0,1]]) ]
gap> [[-47,-37],[61,48]]^phi;
ClassRotation((1,0)+(2,2)Z+(0,3)Z,[[-47,-37],[61,48]])
gap> Display(last:AsTable);

Rcwa permutation of Z^2 with modulus (2,2)Z+(0,3)Z, of order 6

   [m,n] mod (2,2)Z+(0,3)Z   |              Image of [m,n]
-----------------------------+-------------------------------------------
[0,0] [0,1] [0,2] [1,1]     |
[1,2]                       | [m,n]
[1,0]                       | [(-263m+122n+266)/3,(-1147m+532n+1147)/6]
]]>
</Example>
  </Description>
</ManSection>

<Index Key="DrawOrbitPicture" Subkey="for rcwa groups over Z x Z">
  <C>DrawOrbitPicture</C>
</Index>

The function <Ref Func="DrawOrbitPicture"
                  Label="G, p0, bound, h, w, colored, palette, filename"/>
can also be used to depict orbits under the action of rcwa groups
over <M>&ZZ;^2</M>. Further there is a function which depicts
residue class unions of <M>&ZZ;^2</M> and partitions
of <M>&ZZ;^2</M> into such:

<ManSection>
  <Heading> DrawGrid </Heading>
  <Func Name="DrawGrid"
        Arg="U, yrange, xrange, filename"
        Label="U, yrange, xrange, filename"/>
  <Func Name="DrawGrid"
        Arg="P, yrange, xrange, filename"
        Label="P, yrange, xrange, filename"/>
  <Returns>
    nothing.
  </Returns>
  <Description>
    This function depicts the residue class union <A>U</A>
    of <M>&ZZ;^2</M> or the partition <A>P</A>
    of <M>&ZZ;^2</M> into residue class unions, respectively.
    The arguments <A>yrange</A> and <A>xrange</A> are the coordinate
    ranges of the rectangular snippet to be drawn, and the argument
    <A>filename</A> is the name, i.e. the full path name,
    of the output file. If the first argument is a residue class union,
    the output picture is black-and-white, where black pixels represent
    members of <A>U</A> and white pixels represent non-members.
    If the first argument is a partition of <M>&ZZ;^2</M> into residue
    class unions, the produced picture is colored, and different colors are
    used to denote membership in different parts.
  </Description>
</ManSection>

</Section>



</Chapter>

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