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# 6 Ideals, factors, and direct products of nearrings

### Sections

An ideal of a nearring (N,+,*) is a subset I such that I is a normal subgroup of (N,+), and for all iI, n,mN, we have (m+i)*nm*nI and n*iI. Ideals are in one-to-one correspondence to the congruence relations on (N,+,*).

A right ideal of a nearring (N,+,*) is a subset I such that I is a normal subgroup of (N,+), and for all iI, n,mN, we have (m+i)*nm*nI. Right ideals are in one-to-one correspondence to the congruence relations on (N,+, { λm | mM } ), where λm (n) : = n*m. Hence, right ideals describe the congruences of the N-group NN.

A left ideal of a nearring (N,+,*) is a subset I such that I is a normal subgroup of (N,+), and for all iI, nN, we have n*iI.

## 6.1 Direct products

For all sorts of nearrings direct products A ×B can be constructed. The result is again a nearring. In the case that both A and B are `TransformationNearRings`, the result will be a `TransformationNearRing` acting on the direct product of the groups A and B act on. In any other case the result is an `ExplicitMultiplicationNearRing`, even if one of the factors is a `TransformationNearRing`. In any case, the elements of a direct product are not pairs or tuples.

```    gap> A := LibraryNearRing( GTW8_2, 12 );
LibraryNearRing(8/2, 12)
gap> B := LibraryNearRing( GTW12_4, 13 );
LibraryNearRing(12/4, 13)
gap> D := DirectProductNearRing( A, B );
DirectProductNearRing( LibraryNearRing(8/2, 12),
LibraryNearRing(12/4, 13) )
gap> SetName( D, "A x B" );
gap> D;
A x B
```
In this case the result is an `ExplicitMultiplicationNearRing`. It is a good idea to give a shorter name to the nearring D, because we will investigate one of its ideals in the next section.

## 6.2 Ideals and factors

We go on with the last example of the previous section and try to compute a left ideal which is generated by two elements, namely the second and the twenty-fifth in the sorted list of elements. The GAP function `list{[ poss ]}` constructs a list of those elements of the list `list` the position in the list `list` of which is in the list `poss`. For short, `elms{[2,25]}` is a list which contains the second and the twenty-fifth element of the list `elms`.

```    gap> elms := AsSortedList( D );;
gap> gens := elms{[2,25]};
[ (( 8, 9,10)), ((3,5)(4,6)) ]
gap> L := NearRingLeftIdealByGenerators( D, gens );
< nearring left ideal >
```
Now we can start investigating I. We can compute its size and test if it is an ideal.
```    gap> Size( L );
24
gap> IsNearRingRightIdeal( L );
true
gap> L;
< nearring ideal of size 24 >
```
So L is a two-sided ideal with 24 elements. Now we are getting interested in L. Is it a maximal ideal, what is the factor D/L?
```    gap> IsMaximalNearRingIdeal( L );
false
gap> F := D/L;
FactorNearRing( A x B, < nearring ideal of size 24 > )
gap> PrintTable( F, "am" );

+  | n0  n1  n2  n3
--------------------
n0  | n0  n1  n2  n3
n1  | n1  n0  n3  n2
n2  | n2  n3  n0  n1
n3  | n3  n2  n1  n0

*  | n0  n1  n2  n3
--------------------
n0  | n0  n0  n0  n0
n1  | n0  n0  n0  n0
n2  | n0  n0  n0  n0
n3  | n0  n0  n0  n0
```
Here, we use `PrintTable` with a second argument, because we do not want to see all the information. Here `a` stands for addition and `m` stands for multiplication table. For more options see the reference manual. Obviously, F is a constant nearring on a group of order 4. The additive group of the nearring is Z2 ×Z2. To make this fact more obvious, we choose other names (symbols) for the elements of the nearring and print the addition table again.
```    gap> IsElementaryAbelian( GroupReduct( F ) );
true
gap> # this would also convince us
gap> IsCyclic( GroupReduct( F ) );
false
gap> SetSymbols( F, ["(0,0)","(0,1)","(1,0)","(1,1)"] );
gap> PrintTable( F, "m" );

*  | (0,0)  (0,1)  (1,0)  (1,1)
-----------------------------------
(0,0)  | (0,0)  (0,0)  (0,0)  (0,0)
(0,1)  | (0,0)  (0,0)  (0,0)  (0,0)
(1,0)  | (0,0)  (0,0)  (0,0)  (0,0)
(1,1)  | (0,0)  (0,0)  (0,0)  (0,0)
```
So F is the zero-ring on Z2 ×Z2, which is not simple, but we knew that before.

Of course all this operations can be applied to all nearrings.

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SONATA-tutorial manual
December 2022