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# 2 Nearrings

### Sections

1. Defining a nearring multiplication
2. Construction of nearrings
3. Direct products of nearrings
4. Operation tables for nearrings
5. Modified symbols for the operation tables
6. Accessing nearring elements
7. Nearring elements
8. Random nearring elements
9. Nearring generators
10. Size of a nearring
11. The additive group of a nearring
12. Nearring endomorphisms
13. Nearring automorphisms
14. Isomorphic nearrings
15. Subnearrings
16. Invariant subnearrings
17. Constructing subnearrings
18. Intersection of nearrings
19. Identity of a nearring
20. Distributivity in a nearring
21. Elements of a nearring with special properties
22. Special properties of a nearring

A (left) nearring is a nonempty set N together with two binary operations on N, + and · s.t. (N,+) is a group, (N,·) is a semigroup, and · is left distributive over +, i.e. "n1,n2,n3 Î N: n1·(n2+n3) = n1·n2 + n1·n3.

For more information we suggest [Pilz:Nearrings], [meldrum85:NATLWG], and [Clay:Nearrings].

The functions described in this section can be found in the source files `nr.g?` and `nrconstr.g?`.

## 2.1 Defining a nearring multiplication

- `IsNearRingMultiplication(` G`,` mul`, [`lcs`] )`

  The arguments of the function `IsNrMultiplication` are a group G, a GAP-function mul which has two arguments `x` and `y` which must both be elements of the group G and returns an element `z` of G s.t. mul defines a binary operation on G. As an optional third parameter `IsNrMultiplication` accepts a list of control strings lcs.

  `IsNearRingMultiplication` returns `true` (`false`) if mul is (is not) a nearring multiplication on G i.e. it checks whether it is well-defined, associative and left distributive over the group operation of G. The list lcs may contain one or more of the strings `"closed"` , `"ass"` and `"rdistr"` in which case the according property is not tested. In this case it is assumed that the user has checked it. This feature should only be used in cases where it would take too long to check certain laws element by element and the user is absolutely sure about the correctness.

          gap> G := TWGroup( 24, 6 );
          24/6
          gap> mul_l := function ( x, y ) return y; end;
          function ( x, y ) ... end
          gap> IsNearRingMultiplication( G, mul_l );
          true
          gap> mul_r := function ( x, y ) return x; end;
          function ( x, y ) ... end
          gap> IsNearRingMultiplication( G, mul_r );
          #I  specified multiplication is not left distributive.
          false
          gap> IsNearRingMultiplication( G, mul_r, ["closed","ldistr"] );
          true

- `NearRingMultiplicationByOperationTable(` G`,` table`,` elmlist`)`

  The function `NearRingMultiplicationByOperationTable` returns the nearring multiplication on the group G which is defined by the multiplication table table. Rather than group elements the entries of table are the positions of the group element in the list elmlist (the first element in elmlist is 1, the second is 2, a.s.o.). Usually the neutral element of the group will be the first.

  `IsNearRingMultiplication` can be used to check whether the resulting multiplication is indeed a nearring multiplication on G.

          gap> G := CyclicGroup( 4 );
          <pc group of size 4 with 2 generators>
          gap> GeneratorsOfGroup( G );
          [ f1, f2 ]
          gap> a := last[1];
          f1
          gap> Order( a );
          4
          gap> # a generates G indeed
          gap> elmlist := List( [0..3], x -> a^x );
          [ <identity> of ..., f1, f2, f1*f2 ]
          gap> # Let:  1 := identity of ..., 2 := f1, 3 := f2, 4 := f1*f2
          gap> # Consider the following multiplication table on G:
          gap> OT := [[1, 1, 1, 1],
          > [1, 4, 3, 2],
          > [1, 1, 1, 1],
          > [1, 2, 3, 4]];;
              gap> mul := NearRingMultiplicationByOperationTable( G, OT, elmlist );
          function ( x, y ) ... end
          gap> IsNearRingMultiplication( G, mul );
          true

  ## 2.2 Construction of nearrings

- `ExplicitMultiplicationNearRing(` G`,` mul`)`

  The constructor function `ExplicitMultiplicationNearRing` returns the nearring defined by the group G and the nearring multiplication mul. (For a detailed explanation of mul see Section IsNearRingMultiplication.)

  `ExplicitMultiplicationNearRing` calls `IsNearRingMultiplication` in order to make sure that mul is really a nearring multiplication. If the nearring multiplication should not be checked,

- `ExplicitMultiplicationNearRingNC(` G`,` mul`)`

  may be called.

          gap> n := ExplicitMultiplicationNearRing( GTW18_3, mul_l );
          ExplicitMultiplicationNearRing ( 18/3 , multiplication )
          gap> n = ExplicitMultiplicationNearRingNC( GTW18_3, mul_l );
          true

- `IsNearRing(` obj`)`

  `IsNearRing` returns `true` if the object obj is a nearring and `false` otherwise.

          gap> n := ExplicitMultiplicationNearRingNC( GTW18_3, mul_l );
          ExplicitMultiplicationNearRing ( 18/3 , multiplication )
          gap> IsNearRing( n );
          true
          gap> IsNearRing( GroupReduct( n ) );
          false

- `IsExplicitMultiplicationNearRing(` obj`)`

  `IsExplicitMultiplicationNearRing` returns `true`, if the object obj is a nearring defined by a group and a multiplication as with ExplicitMultiplicationNearRing.

          gap> IsExplicitMultiplicationNearRing( n );
          true

  ## 2.3 Direct products of nearrings

- `DirectProductNearRing(` nr1`,` nr2`)`

  Given two nearrings nr1 and nr2, the function `DirectProductNearRing` constructs the direct product of these.

          gap> n := ExplicitMultiplicationNearRingNC( GTW18_3, mul_l );
          ExplicitMultiplicationNearRing ( 18/3 , multiplication )
          gap> zero_mul := function ( x, y ) return (); end;
          function ( x, y ) ... end
          gap> z := ExplicitMultiplicationNearRingNC( GTW12_3, zero_mul );
          ExplicitMultiplicationNearRing ( 12/3 , multiplication )
          gap> d := DirectProductNearRing( n, z );
          DirectProductNearRing( ExplicitMultiplicationNearRing ( 18/3 , multi\
          plication ), ExplicitMultiplicationNearRing ( 12/3 , multiplication \
          ) )
          gap> IsExplicitMultiplicationNearRing( d );
          true

  ## 2.4 Operation tables for nearrings

- `PrintTable(` nr`)`

  `PrintTable` prints the additive and multiplicative Cayley tables of the nearring nr. This function works the same way as for groups.

          gap> n := ExplicitMultiplicationNearRingNC( CyclicGroup( 3 ), mul_l );
          ExplicitMultiplicationNearRing ( <pc group of size 3 with
          1 generators> , multiplication )
          gap> SetSymbols( n, ["0","1","2"] );
          gap> PrintTable( n );
          Let:
          0 := (<identity> of ...)
          1 := (f1)
          2 := (f1^2)

            +  | 0  1  2
            ------------
            0  | 0  1  2
            1  | 1  2  0
            2  | 2  0  1

            *  | 0  1  2
            ------------
            0  | 0  1  2
            1  | 0  1  2
            2  | 0  1  2

  Optionally, `PrintTable` can be used in the form `PrintTable(` nr`,` mode`)`, where mode is a string. If the letter `e` is contained in this string, the definitions of the symbols used are printed, if the letter `a` is contained in the string, the addition table is printed, and if the letter `m` is contained in the string, the multiplication table of the nearring is printed. Every combination of these three letters in any order is possible.

  ## 2.5 Modified symbols for the operation tables

- `SetSymbols(` nr`,` symblist`)`
- `SetSymbolsSupervised(` nr`,` symblist`)`

  The function `SetSymbols` and `SetSymbolsSupervised` allow you to define a list symblist of strings to be used when printing the operation tables of the nearring. `SetSymbols` simply sets the set of strings to the given value. `SetSymbolsSupervised` checks, if there are more symbols than the nearring has elements. In this case the superfluous strings are ignored. If there are less symbols than the nearring has elements, `SetSymbolsSupervised` ``invents'' unique names for the rest of the elements. In any case a warning is printed. If there are repetitions or holes in the list symblist an error is signaled.

- `Symbols(` nr`)`

  allows you to look at the set of symbols, which are currently in use.

          gap> n := LibraryNearRing( GTW3_1, 4 );
          LibraryNearRing(3/1, 4)
          gap> Symbols( n );
          [ "n0", "n1", "n2" ]
          gap> SetSymbolsSupervised( n,
                  ["apple", "banana", "coconut", "donut", "potato"] );
          Warning: too many symbols ...ignoring the last 2 symbols
          gap> PrintTable( n, "m" );

                  *  | apple    banana   coconut
            ------------------------------------
            apple    | apple    apple    apple
            banana   | apple    banana   coconut
            coconut  | apple    banana   coconut

  ## 2.6 Accessing nearring elements

  The elements of a nearring are different from those of its group reduct. In order to make group elements and nearring elements distinguishable for the user, nearring elements are printed with an extra pair of parentheses. The two functions `AsGroupReductElement` and `AsNearRingElement` can be used to switch between these two representations.

- `AsNearRingElement(` nr`,` grpelm`)`

  returns the representation as a nearring element of an element grpelm of the group reduct of the nearring nr.

- `AsGroupReductElement(` nrelm`)`

  returns the representation as an element of the group reduct of the nearring of the nearring element nrelm.

          gap> mul_l := function ( x, y ) return y; end;
          function ( x, y ) ... end
          gap> n := ExplicitMultiplicationNearRingNC( GTW6_2, mul_l );
          ExplicitMultiplicationNearRing ( 6/2 , multiplication )
          gap> AsList( n );
          [ (()), ((2,3)), ((1,2)), ((1,2,3)), ((1,3,2)), ((1,3)) ]
          gap> e := AsNearRingElement( n, (2,3) );
          ((2,3))
          gap> e in n;
          true
          gap> f := AsNearRingElement( n, (1,3) );
          ((1,3))
          gap> e + f;
          ((1,3,2))
          gap> e * f;
          ((1,3))
          gap> p := AsGroupReductElement( e );
          (2,3)
          gap> IsPerm( p );
          true
          gap> p + p;
          Error no method found for operation SUM with 2 arguments at
          Error( "no method found for operation ", NAME_FUNC( operation ),
           " with 2 arguments" );
          Entering break read-eval-print loop, you can 'quit;' to quit to outer l\
          oop,
          or you can return to continue
          brk>

  ## 2.7 Nearring elements

  There are three different ways to ask for the elements of a nearring.

- `AsList(` nr`)`

  The function `AsList` computes the elements of the nearring nr. It returns the elements as a list.

- `AsSortedList(` nr`)`

  does essentially the same, but returns a set of elements.

- `Enumerator(` nr`)`

  does essentially the same as `AsList`, but returns an enumerator for the elements of nr. An enumerator is an object that is capable of enumerating the elements the nearring one by one. This is especially important if the nearring is very big and not every element can be stored.

          gap> n := LibraryNearRing( GTW6_2, 39 );
          LibraryNearRing(6/2, 39)
          gap> e := Enumerator( n );
          <enumerator of near ring>
          gap> e[1];
          (())
          gap> x := AsNearRingElement( n, (1,2,3) );
          ((1,2,3))
          gap> Position( e, x );
          2
          gap> Length(e);
          6
          gap> l := AsList( n );
          [ (()), ((2,3)), ((1,2)), ((1,2,3)), ((1,3,2)), ((1,3)) ]
          gap> e[3] = l[3];
          false
          gap> AsSortedList( n );
          [ (()), ((2,3)), ((1,2)), ((1,2,3)), ((1,3,2)), ((1,3)) ]

  ## 2.8 Random nearring elements

- `Random(` nr`)`

  `Random` returns a random element of the nearring nr.

          gap> n := LibraryNearRing( GTW6_2, 39 );
          LibraryNearRing(6/2, 39)
          gap> Random(n);
          ((1,3))

  ## 2.9 Nearring generators

- `GeneratorsOfNearRing(` nr`)`

  The function `GeneratorsOfNearRing` returns a set of (not necessarily additive) generators of the nearring nr.

          gap> n := ExplicitMultiplicationNearRingNC( GTW8_4, mul_l );
          ExplicitMultiplicationNearRing ( 8/4 , multiplication )
          gap> GeneratorsOfNearRing( n );
          [ ((1,2,3,4)), ((2,4)) ]

  ## 2.10 Size of a nearring

- `Size(` nr`)`

  `Size` returns the number of elements in the nearring nr.

          gap> n := LibraryNearRingWithOne( GTW24_3, 8 );
          LibraryNearRingWithOne(24/3, 8)
          gap> Size(n);
          24

  ## 2.11 The additive group of a nearring

- `GroupReduct(` nr`)`

  The function `GroupReduct` returns the nearring nr as a (multiplicative) group.

          gap> GroupReduct( LibraryNearRingWithOne( GTW24_3, 8 ) );
          24/3

  ## 2.12 Nearring endomorphisms

- `Endomorphisms(` nr`)`

  `Endomorphisms` computes all the endomorphisms of the nearring nr. The endomorphisms are returned as a list of transformations. In fact, the returned list contains those endomorphisms of the group reduct of nr which are also nearring endomorphisms.

          gap> Endomorphisms ( LibraryNearRing( GTW12_4, 4 ) ) ;
          [ [ (1,2,4), (2,3,4) ] -> [ (), () ],
            [ (1,2,4), (2,3,4) ] -> [ (1,2,4), (2,3,4) ] ]
          gap> Length( Endomorphisms( GTW12_4 ) );
          33

  ## 2.13 Nearring automorphisms

- `Automorphisms(` nr`)`

  `Automorphisms` computes all the automorphisms of the nearring nr. The automorphisms are returned as a list of transformations. In fact, the returned list contains those automorphisms of the group reduct of nr which are also nearring automorphisms.

          gap> Automorphisms( LibraryNearRing( GTW12_4, 4 ) );
          [ IdentityMapping( 12/4 ) ]

  ## 2.14 Isomorphic nearrings

- `IsIsomorphicNearRing(` nr1`,` nr2`)`

  The function `IsIsomorphicNearRing` returns `true` if the two nearrings nr1 and nr2 are isomorphic and `false` otherwise.

          gap> IsIsomorphicNearRing( MapNearRing( GTW2_1 ),
          > LibraryNearRingWithOne( GTW4_2, 5 ) );
          true

  ## 2.15 Subnearrings

- `SubNearRings(` nr`)`

  The function `SubNearRings` computes all subnearrings of the nearring nr. The function returns a list of nearrings representing the according subnearrings.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> SubNearRings( n );
          [ ExplicitMultiplicationNearRing ( Group(()) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (1,4)(2,3) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (1,2)(3,4) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (1,4)(2,3), (1,3)(2,4)
               ]) , multiplication ), ExplicitMultiplicationNearRing ( Group(
              [ (1,4)(2,3), (1,3)(2,4), (1,2)(3,4), (2,3,4) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (2,3,4) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (1,4,3) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (1,2,4) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (1,3,2) ]) , multiplication ) ]

  ## 2.16 Invariant subnearrings

- `InvariantSubNearRings(` nr`)`

  A subnearring (M,+,·) of a nearring (N,+,·) is called an invariant subnearring if both, M ·N and N ·M are subsets of M.

  The function `InvariantSubNearRings` computes all invariant subnearrings of the nearring nr. The function returns a list of nearrings representing the according invariant subnearrings.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> i := InvariantSubNearRings( n );
          [ ExplicitMultiplicationNearRing ( Group(()) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (1,2)(3,4) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group(
              [ (1,4)(2,3), (1,3)(2,4), (1,2)(3,4), (2,3,4) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (2,3,4) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group([ (1,4,3) ]) , multiplication ) ]

  ## 2.17 Constructing subnearrings

- `SubNearRingBySubgroupNC(` nr`,` S`)`

  For a subgroup S of the group reduct of the nearring which is closed under the multiplication of nr the function `SubNearRingBySubgroupNC` returns the subnearRing of nr, which is induced by this subgroup. The nr-invariance is not expli[ly] tested.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> S := Subgroup( GTW12_4, [ (1,2)(3,4) ] );
          Group([ (1,2)(3,4) ])
          gap> sn := SubNearRingBySubgroupNC( n, S );
          ExplicitMultiplicationNearRing ( Group([ (1,2)(3,4) ]) , multiplication )

  ## 2.18 Intersection of nearrings

- `Intersection(` listofnrs`)`

  computes the intersection of the nearrings in the list listofnrs. All of the nearrings in listofnrs must be subnearrings of a common supernearring.

          gap> n := LibraryNearRingWithOne( GTW27_4, 5 );
          LibraryNearRingWithOne(27/4, 5)
          gap> si := Filtered( SubNearRings( n ), s -> Identity( n ) in s );
          [ ExplicitMultiplicationNearRing ( Group(
              [ ( 1,23,14)( 2,13, 6)( 3,27,22)( 4,18, 9)( 5,20,12)( 7,16,26)
                  ( 8,25,17)(10,21,19)(11,24,15) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group(
              [ ( 1,26,27)( 2,19,20)( 3,14,16)( 4,24,25)( 5, 6,21)( 7,22,23)
                  ( 8, 9,11)(10,12,13)(15,17,18),
                ( 1,22,16)( 2,12,21)( 3,26,23)( 4,17,11)( 5,19,13)( 6,20,10)
                  ( 7,14,27)( 8,24,18)( 9,25,15) ]) , multiplication ),
            ExplicitMultiplicationNearRing ( Group(
              [ ( 1, 6,15)( 2, 9, 7)( 3,13,25)( 4,14,10)( 5,18,27)( 8,23,20)
                  (11,22,19)(12,24,16)(17,26,21),
                ( 1, 5,17)( 2, 8,22)( 3,12, 4)( 6,18,26)( 7,20,11)( 9,23,19)
                  (10,25,16)(13,24,14)(15,27,21),
                ( 1, 2, 4)( 3, 6,11)( 5, 9,16)( 7,13,17)( 8,14,21)(10,18,22)
                  (12,15,23)(19,24,26)(20,25,27) ]) , multiplication ) ]
          gap> Intersection( si );
          ExplicitMultiplicationNearRing ( Group(
          [ ( 1,23,14)( 2,13, 6)( 3,27,22)( 4,18, 9)( 5,20,12)( 7,16,26)
              ( 8,25,17)(10,21,19)(11,24,15) ]) , multiplication )
          gap> Size( last );
          3

  ## 2.19 Identity of a nearring

- `Identity(` nr`)`

  The function `Identity` returns the identity of the multiplicative semigroup of the nearring nr if it exists and `fail` otherwise.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> Identity( n );
          fail
          gap> n := LibraryNearRingWithOne( GTW24_4, 8 );
          LibraryNearRingWithOne(24/4, 8)
          gap> Identity( n );
          ((1,2,3,4,5,6)(7,8))

- `IsNearRingWithOne(` nr`)`

  The function `IsNearRingWithOne` returns `true` if the nearring was constructed as a nearring with one and `false` otherwise. To decide whether a nearring has an identity use `Identity(`nr`)=true`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsNearRingWithOne( n );
          false
          gap> n := LibraryNearRingWithOne( GTW24_4, 8 );
          LibraryNearRingWithOne(24/4, 8)
          gap> Identity( n );
          ((1,2,3,4,5,6)(7,8))
          gap> IsNearRingWithOne( n );
          true

  ## 2.20 Distributivity in a nearring

- `Distributors(` nr`)`

  An element x of a nearring (N,+,·) is called a

  distributor if x = (n1 + n2 ) ·n3 - (n1 ·n3 + n2 ·n3 ) for some elements n1, n2, n3 of N.

  The function `Distributors` returns a list containing the distributors of the nearring nr.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsNearRingWithOne( n );
          false
          gap> Distributors( n );
          [ (()), ((2,3,4)), ((2,4,3)), ((1,2)(3,4)), ((1,2,3)), ((1,2,4)),
            ((1,3,2)), ((1,3,4)), ((1,3)(2,4)), ((1,4,2)), ((1,4,3)),
            ((1,4)(2,3)) ]

- `DistributiveElements(` nr`)`

  An element d of a left nearring (N,+,·) is called a distributive element if it is also right distributive over all elements, i.e. "n1, n2 Î N: (n1 + n2 )·d = n1·d + n2·d.

  The function `DistributiveElements` returns a list containing the distributive elements of the nearring nr.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> DistributiveElements( n );
          [ (()) ]

- `IsDistributiveNearRing(` nr`)`

  A left nearring N is called distributive nearring if its multiplication is also right distributive.

  The function `IsDistributiveNearRing` simply checks if all elements are distributive and returns the according boolean value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsDistributiveNearRing( n );
          false

  ## 2.21 Elements of a nearring with special properties

- `ZeroSymmetricElements(` nr`)`

  Let (N,+,·) be a left nearring and denote by 0 the neutral element of (N,+). An element n of N is called a zero-symmetric element if 0·n = 0.

  Remark: note that in a left nearring n·0 = 0 is true for all elements n.

  The function `ZeroSymmetricElements` returns a list containing the zero-symmetric elements of the nearring nr.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> ZeroSymmetricElements( n );
          [ (()), ((2,3,4)), ((2,4,3)), ((1,2)(3,4)), ((1,2,3)), ((1,2,4)),
            ((1,3,2)), ((1,3,4)), ((1,3)(2,4)), ((1,4,2)), ((1,4,3)),
            ((1,4)(2,3)) ]

- `IdempotentElements(` nr`)`

  The function `IdempotentElements` returns a list containing the idempotent elements of the multiplicative semigroup of the nearring nr.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IdempotentElements( n );
          [ (()), ((1,4)(2,3)) ]

- `NilpotentElements(` nr`)`

  Let (N,+,·) be a nearring with zero 0. An element n of N is called nilpotent if there is a positive integer k such that nk = 0.

  The function `NilpotentElements` returns a list of sublists of length 2 where the first entry is a nilpotent element n and the second entry is the smallest k such that nk = 0.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> NilpotentElements( n );
          [ [ (()), 1 ], [ ((2,3,4)), 2 ], [ ((2,4,3)), 2 ],
            [ ((1,2)(3,4)), 2 ], [ ((1,2,3)), 2 ], [ ((1,2,4)), 2 ],
            [ ((1,3,2)), 2 ], [ ((1,3,4)), 2 ], [ ((1,4,2)), 2 ],
            [ ((1,4,3)), 2 ] ]

- `QuasiregularElements(` nr`)`

  Let (N,+,·) be a left nearring. For an element z Î N, denote the right ideal generated by the set {n - z·n | n Î N} by Lz. An element z of N is called quasiregular if z Î Lz.

  The function `QuasiregularElements` returns a list of all quasiregular elements of a nearring nr.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> QuasiregularElements( n );
          [ (()), ((2,3,4)), ((2,4,3)), ((1,2)(3,4)), ((1,2,3)), ((1,2,4)),
            ((1,3,2)), ((1,3,4)), ((1,3)(2,4)), ((1,4,2)), ((1,4,3)) ]

- `RegularElements(` nr`)`

  Let (N,+,·) be a nearring. An element n of N is called regular if there is an element x such that n·x·n = n.

  The function `RegularElements` returns a list of all regular elements of a nearring nr.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> RegularElements( n );
          [ (()), ((1,3)(2,4)), ((1,4)(2,3)) ]

  ## 2.22 Special properties of a nearring

- `IsAbelianNearRing(` nr`)`

  A nearring is called abelian if its group reduct is abelian.

  The function `IsAbelianNearRing` returns the according boolean value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsAbelianNearRing( n );
          false

- `IsAbstractAffineNearRing(` nr`)`

  A left nearring N is called abstract affine if its group reduct is abelian and its zero-symmetric elements are exactly its distributive elements.

  The function `IsAbstractAffineNearRing` returns the according boolean value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsAbstractAffineNearRing( n );
          false

- `IsBooleanNearRing(` nr`)`

  A left nearring N is called boolean if all its elements are idempotent with respect to multiplication.

  The function `IsBooleanNearRing` simply checks if all elements are idempotent and returns the according boolean value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsBooleanNearRing( n );
          false

- `IsNilNearRing(` nr`)`

  A nearring N is called nil if all its elements are nilpotent.

  The function `IsNilNearRing` checks if all elements are nilpotent and returns the according boolean value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsNilNearRing( n );
          false

- `IsNilpotentNearRing(` nr`)`

  A nearring N is called nilpotent if there is a positive integer k, s.t. Nk = {0}.

  The function `IsNilpotentNearRing` tests if the nearring nr is nilpotent and returns the according boolean value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsNilpotentNearRing( n );
          false

- `IsNilpotentFreeNearRing(` nr`)`

  A nearring N is called nilpotent free if its only nilpotent element is 0.

  The function `IsNilpotentFreeNearRing` checks if 0 is the only nilpotent and returns the according boolean value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsNilpotentFreeNearRing( n );
          false

- `IsCommutative(` nr`)`

  A nearring (N,+,·) is called commutative if its multiplicative semigroup is commutative.

  The function `IsCommutative` returns the according value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsCommutative( n );
          false

- `IsDgNearRing(` nr`)`

  A nearring (N,+,·) is called distributively generated (d.g.) if (N,+) is generated additively by the distributive elements of the nearring.

  The function `IsDgNearRing` returns the according value `true` or `false` for a nearring nr.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsDgNearRing( n );
          false

- `IsIntegralNearRing(` nr`)`

  A nearring (N,+,·) with zero element 0 is called integral if it has no zero divisors, i.e. the condition "n1,n2: n1 ·n2 = 0 Þ n1 = 0 Ú n2 = 0 holds.

  The function `IsIntegralNearRing` returns the according value `true` or `false` for a nearring nr.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsIntegralNearRing( n );
          false

- `IsPrimeNearRing(` nr`)`

  A nearring (N,+,·) with zero element 0 is called prime if the ideal { 0 } is a prime ideal.

  The function `IsPrimeNearRing` checks if nr is a prime nearring by using the condition for all non-zero ideals I,J: I ·J ¹ { 0 } and returns the according value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsPrimeNearRing( n );
          true

- `IsQuasiregularNearRing(` nr`)`

  A nearring N is called quasiregular if all its elements are quasiregular.

  The function `IsQuasiregularNearRing` simply checks if all elements of the nearring nr are quasiregular and returns the according boolean value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsQuasiregularNearRing( n );
          false

- `IsRegularNearRing(` nr`)`

  A nearring N is called regular if all its elements are regular.

  The function `IsRegularNearRing` simply checks if all elements of the nearring nr are regular and returns the according boolean value `true` or `false`.

          gap> n := LibraryNearRing( GTW12_4, 8 );
          LibraryNearRing(12/4, 8)
          gap> IsRegularNearRing( n );
          false

- `IsNearField(` nr`)`

  Let (N,+,·) be a nearring with zero 0 and denote by N* the set N - {0}. N is a nearfield if (N,+,·) has an identity and (N*,·) is a group.

  The function `IsNearField` tests if nr has an identity and if every non-zero element has a multiplicative inverse and returns the according value `true` or `false`.

           gap> n := LibraryNearRing( GTW12_4, 8 );
           LibraryNearRing(12/4, 8)
           gap> IsNearField( n );
           false

- `IsPlanarNearRing(` nr`)`

  Let (N,+,·) be a left nearring. For a,b Î N we define a º b iff a·n = b·n for all n Î N. If a º b, then a and b are called equivalent multipliers. A nearring N is called planar if | N/ º | ³ 3 and if for any two non-equivalent multipliers a and b in N, for any c Î N, the equation a·x = b·x + c has a unique solution.

  The function `IsPlanarNearRing` returns the according value `true` or `false` for a nearring nr.

           gap> n := LibraryNearRing( GTW9_2, 90 );
           LibraryNearRing(9/2, 90)
           gap> IsPlanarNearRing( n );
           true

- `IsWdNearRing(` nr`)`

  A left nearring (N,+,·) is called weakly divisible if "a,b Î N $x Î N : a·x = b or b·x = a.

  The function `IsWdNearRing` returns the according value true or false for the nearring nr.

           gap> nr := LibraryNearRing( GTW9_1, 185 );
           LibraryNearRing(9/1, 185)
           gap> IsWdNearRing( nr );
           true

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