There are many non-isomorphic nearrings, even of small order.
All non-isomorphic nearrings of orders `2` to `15` and
all non-isomorphic nearrings with identity up
to order `31` with exception of those on
the elementary abelian groups of orders
`16` and `27` are collected in the SONATA
nearring library.

The number of nearrings in the library is big. For example, try

gap> NumberLibraryNearRings( GTW12_3 ); 48137

Try your favorite small groups with this function to get an impression of these numbers.

Of course, no one can know all these nearrings personally. Therefore, the main purpose of the nearring library is to filter out the nearrings of interest.

Consider for example the following

**Problem:** How many non-rings with identity of order `4` are
there and what do they look like? If you cannot answer this
question adhoc, stay tuned.

Let's start with the groups of order `4`. Of course you know,
there are `2` groups of order `4`: `GTW4_1`

, the cyclic group
and `GTW4_2`

, Klein's four group.

Let's go for `GTW4_1`

first:

gap> NumberLibraryNearRingsWithOne( GTW4_1 ); 1 gap> Filtered( AllLibraryNearRingsWithOne( GTW4_1 ), > n -> not IsDistributiveNearRing( n ) ); [ ]

So, the only nearring with identity there is
on `GTW4_1`

is the ring. Well... you knew that before,
didn't you?

Now for `GTW4_2`

:

gap> NumberLibraryNearRingsWithOne( GTW4_2 ); 5 gap> Filtered( AllLibraryNearRingsWithOne( GTW4_2 ), > n -> not IsDistributiveNearRing( n ) ); [ LibraryNearRing(4/2, 12), LibraryNearRing(4/2, 22) ]

Here we go:

gap> PrintTable( LibraryNearRing( GTW4_2, 12 ) ); Let: n0 := (()) n1 := ((3,4)) n2 := ((1,2)) n3 := ((1,2)(3,4)) + | 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 n1 n1 n2 | n0 n0 n2 n2 n3 | n0 n1 n2 n3 gap> PrintTable( LibraryNearRing( GTW4_2, 22 ) ); Let: n0 := (()) n1 := ((3,4)) n2 := ((1,2)) n3 := ((1,2)(3,4)) + | 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 n2 n2 n1 | n0 n1 n2 n3 n2 | n0 n2 n2 n0 n3 | n0 n3 n2 n1

An alternative to filtering the nearring library is to
use a `for ... do ... od`

construction.

We shall demonstrate this by recomputing the list
of nearrings given in appendix K of <[>Pilz:Nearrings],
i.e. a list of all nearrings on the dihedral group of order `8`
(`GTW8_4`

) which have an identity, are non-zerosymmetric or
are integral.

First, we initialize the variable `nr_list`

as the empty list:

gap> nr_list := [ ]; [ ]

Now, we write ourselves a `for`

loop and add those nearrings
we want:

gap> for i in [1..NumberLibraryNearRings( GTW8_4 )] do > n := LibraryNearRing( GTW8_4, i ); > if ( not IsZeroSymmetricNearRing( n ) or > IsIntegralNearRing( n ) or > Identity( n ) <> fail > ) then > Add( nr_list, n ); > fi; > od; gap> Length( nr_list ); 141

How many boolean nearrings are amongst these? We call a nearring
**boolean** if `x*x=x` for all `x inN`.

gap> Filtered( nr_list, IsBooleanNearRing ); [ LibraryNearRing(8/4, 1314), LibraryNearRing(8/4, 1380), LibraryNearRing(8/4, 1446), LibraryNearRing(8/4, 1447) ]

Which correspond to the numbers
`140`, `86`, `99`, and `141` in
<[>Pilz:Nearrings], appendix K, accordingly.

For those who got interested in boolean nearrings: many results about them have been collected in <[>Pilz:Nearrings], 9.31.

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SONATA-tutorial manual

November 2012