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# 8 Designs

### Sections

1. Designs from planar nearrings
2. Information on designs

Various designs can be obtained from nearrings. The design structure and basic functions for the manipulation of designs have been implemented within SONATA, as well as the means to construct the most popular nearring designs. Please see the reference manual for other ways to generate designs, e.g. by a set of blocks or by an incidence matrix.

## 8.1 Designs from planar nearrings

We are going to generate a design with a feasible parameter set from a planar nearring, respectively from a Ferrero pair. For notations and definitions we refer to <[>Clay:Nearrings].

Let N be a (left) planar nearring. Then we can define a design (N,B*, ∈ ) with N as the set of points and {N*·a+b | a,b ∈ N, a ≠ 0 } as set of blocks. Here N* = { x ∈ N | x·N = N }. We note that such a design is always a BIB-design. Suppose that the planar nearring N is obtained from the Ferrero pair (G,Φ) as described in the previous chapter with v : = |G|, k : = |Φ|. We can identify G and the additive group of the nearring N. Then N*·a = Φ(a) for all a ∈ N. The number of points of (N,B*, ∈ ) is v, each block has size k, and any 2 distinct points are together incident with precisely k−1 blocks. We say that (N,B*, ∈ ) is a 2−(v,k,k−1) design. Note that k divides v−1.

The problem: Find a 2−(16,5,4) design (N,B*, ∈ ) if possible.

The solution: Since our design should have 16 points, we need a planar nearring of size 16. Furthermore, this planar nearring has an underlying Ferrero pair with a fixed-point-free (fpf) automorphism group of order 5, since the blocks should be of that size.

Let (G,Φ) be such a Ferrero pair. We note that Φ is fpf on all characteristic subgroups of G, in particular, on the center of G. For our example the size of the center modulo 5 has to be congruent to 1. Therefore the center has order 16, and G is abelian. Since Φ is also fpf on any factor of G by a characteristic subgroup, Φ is fpf on the factor of G by its Frattini subgroup. Thus G is elementary abelian.

Now we compute an fpf automorphism group of order 5 on the elementary abelian group of order 16:

        gap> aux := FpfAutomorphismGroupsCyclic( [2,2,2,2], 5 );
        [ [ [ f1, f2, f3, f4 ] -> [ f4, f1*f2, f2*f3, f3*f4 ] ],
          <pc group of size 16 with 4 generators> ]
        gap> a := aux[1][1];
        [ f1, f2, f3, f4 ] -> [ f4, f1*f2, f2*f3, f3*f4 ]
        gap> phi := Group( a );;
        gap> G := aux[2];;

Up to conjugacy there is only one fpf automorphism group of order 5. We note that all planar nearrings obtained from a fixed Ferrero pair (G,Φ) yield the same design. We build the blocks Φ(a)+b for a,b ∈ G, a ≠ 0, of (N,B*, ∈ ) directly from (G,Φ) without actually generating a nearring.

        gap> D := DesignFromFerreroPair( G, phi, "*" );
        <a 2 - ( 16, 5, 4 ) nearring generated design>

The reference manual describes other options for `DesignFromFerreroPair` besides `"*"` and the correponding designs as well.

## 8.2 Information on designs

We investigate the design `D`, which we have generated from a planar nearring in the last section.

        gap> D;
        <a 2 - ( 16, 5, 4 ) nearring generated design>
        gap> DesignParameter( D );
        [ 2, 16, 48, 15, 5, 4 ]

`DesignParameter( D )` returns the set of parameters t, v, b, r, k, λ of the design `D`. Here there are 16 points, 48 blocks, every point is incident with precisely 15 blocks, every block is incident with precisely 5 points, every 2 distinct points are together incident with precisely 4 blocks. The design can be visualized by printing the incidence matrix. The rows are labelled by the points, the columns by the blocks. The point of number i is incident with the block of number j if and only if the entry in the i-th row, j-th column is 1.

        gap> PrintIncidenceMat( D );
        ...1...1...11...1...1..1..11..1...1..1..1...11..
        1......1...1..1.1...11...1..1..1...11..1..1....1
        .1..1......11..1...1..1...11....1..1..11..1...1.
        ..1..1..1...1...1.1...1..1...11...1.1....11...1.
        1....11..1......1..1.1....1.1...11...1...1..1.1.
        .1..1.1...1..1......11....1..11....11....1.1.1..
        ..1..1.1.1...1...1...1...1.1....1.1...11...1.1..
        ..11...1..1.1..1..1.....1...1...1..1.1...1.1...1
        ..1.1...1.1...1..1.1.1.....1..1..1...1..1.1....1
        1...1.1....1.1...11...1.1.......1.1.1...1...1..1
        1...1...11....11....1..11....1....1..1.1...1..1.
        .1...1..1.1.1....1.1...11...1..1....1..1....11..
        .1.1....11...1.1....1.1..1.1...1.1.......1..1..1
        .1.1..1....1..1..11....1.1..1.1..1....1....1..1.
        ..11..1..1....1.1..1..1.1....1.1...1..1.1....1..
        1....1.1..1..1.1..1....1..1..1.1.1....1.1.1.....

By checking the entries in the incidence matrix we realize this design is circular, that is, any 2 distinct blocks have at most 2 points in common. Actually, this is not so easy to see. We prefer to do it like this:

        gap> IsCircularDesign( D );
        true

There are convenient functions to check which points are incident with a given set of blocks and vice versa.

        gap> PointsIncidentBlocks( D, [2,7] );
        [ 6, 14 ]
        gap> BlocksIncidentPoints( D, [6,14] );
        [ 2, 7, 31, 44 ]

Here the 6-th and the 14-th point are incident with the given blocks with numbers 2 and 7. The numbering corresponds to that of the columns and rows of the incidence matrix. Blocks 2, 7, 31, and 44 are incident with points 6 and 14.

The cardinalities of the intersections of one block with all the others, e.g. of the 4-th block with all 48 blocks are given like this.

        gap> BlockIntersectionNumbers( D, 4 );
        [ 0, 2, 2, 5, 0, 0, 2, 2, 1, 2, 1, 2, 2, 1, 2, 2, 2, 1, 2, 1, 2, 0,
          2, 2, 2, 2, 1, 2, 2, 1, 2, 2, 1, 2, 1, 2, 0, 2, 2, 0, 2, 2, 0, 2,
          2, 2, 1, 2 ]

Of course, all these functions can be applied to all kinds of designs no matter how they have been generated.

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