The same approach is used as when k=2, namely, to classify the points and lines, and then to consider removing parts of the geometry to get other kinds of designs.
The starting point is a BIBD with λ=1, obtained by considering just the lines and points of the geometry. There are (q12ν-1)/(q3ν-1) = q9ν+q6ν+q3ν+1 points, ((q12ν-1)(q9ν-1))/((q6ν-1)(q3ν-1))= (q6ν+1)(q6ν+q3ν+1) lines, (q3ν+1) points on a line, and (q9ν-1)/(q3ν-1)= (q6ν+q3ν+1) lines on a point.
There are secants as before (denoted by s), but two sorts of tangents (denoted by t1 and t2) and exterior lines (denoted by x1 and x2).
The number of points in the sub-geometry (C0) is (q4ν-1)/(qν-1)= q3ν+q2ν+qν+1, and there are (qν+1) points on a line, and (q3ν-1)/(qν-1)=q2ν+qν+1 lines on a point, in the sub-geometry.
The matrix showing the numbers of points per line, and vice versa, class by class is:
| (q2ν+1)(q2ν+qν+1) | qν(q4ν-1)(q2ν+qν+1)(qν+1) | q3ν(q4ν-1)(q2ν-1) | q3ν(q4ν-1)(q2ν-1) | q5ν(q4ν-1)(q3ν-1) | ||
|---|---|---|---|---|---|---|
| s | t1 | t2 | x1 | x2 | ||
| q3ν+q2ν+qν+1 | C0 | qν+1,q2ν+qν+1 | 1,(q2ν+qν+1)(q3ν-qν) | 1,q3ν(q2ν-1)(qν-1) | 0,0 | 0,0 |
| (q3ν-qν)(q2ν+1)(q2ν+qν+1) | C1 | q3ν-qν,1 | q2ν,q2ν(qν+1) | 0,0 | q2ν+qν+1,q2ν(q2ν-1) | qν+1,q4ν(q2ν-1) |
| q3ν(q4ν-1)(q2ν-1) | C2 | 0,0 | q3ν-q2ν,q2ν+qν+1 | q3ν,q3ν | q3ν-q2ν-qν,q3ν-q2ν-qν | q3ν-qν,q6ν-q3ν |
The complete design above is a BIBD with λ=1.
Some of the PBDs which can be obtained from this geometry are discussed here.
This table, like the rest of the information on this topic, has been obtained by straightforward counting arguments.
The numbers of secants, the total number of tangents, and the total number of exterior lines are given by the parameters of the two geometries, as are the numbers of points in the three classes. The structure of the secants is likewise given, and the structures of one type of tangent and of one type of exterior line are given by considering the plane of type B0, discussed here. The structure of the second type of tangent, as is the number of such lines, is then determined. The information thus far obtained is then sufficient to get the rest of the matrix, using a set of simultaneous equations.
There are similar matrices, which can mostly be obtained from the information already given, relating points to planes, and planes to lines. The latter is (more-or-less) the dual of the matrix above.
The matrix showing the numbers of points per plane, and vice versa, class by class is:
| q3ν+q2ν+qν+1 | (q3ν-qν)(q2ν+1)(q2ν+qν+1) | q3ν(q4ν-1)(q2ν-1) | ||
|---|---|---|---|---|
| B0 | B1 | B2 | ||
| q3ν+q2ν+qν+1 | C0 | q2ν+qν+1,q2ν+qν+1 | qν+1,qν(q2ν-1)(q2ν+qν+1) | 1,q3ν(q2ν-1)(qν-1) |
| (q3ν-qν)(q2ν+1)(q2ν+qν+1) | C1 | qν(q3ν-1)(qν+1),qν+1 | qν(q3ν+q2ν-1),qν(q3ν+q2ν-1) | q2ν(q2ν+qν+1),q4ν(q2ν-1)) |
| q3ν(q4ν-1)(q2ν-1) | C2 | q3ν(q2ν-1)(qν-1),1 | q4ν(q2ν-1),q2ν(q2ν+qν+1) | q2ν(q4νq2ν-1),q2ν(q4νq2ν-1) |
The complete design above is a BIBD with λ=q3ν+1.
The matrix showing the numbers of planes through a line, and the number of lines in a plane, class by class is:
| (q2ν+1)(q2ν+qν+1) | qν(q4ν-1)(q2ν+qν+1)(qν+1) | q3ν(q4ν-1)(q2ν-1) | q3ν(q4ν-1)(q2ν-1) | q5ν(q4ν-1)(q3ν-1) | ||
|---|---|---|---|---|---|---|
| s | t1 | t2 | x1 | x2 | ||
| q3ν+q2ν+qν+1 | B0 | qν+1,q2ν+qν+1 | 1,(q2ν+qν+1)(q3ν-qν) | 0,0 | 1,q3ν(q2ν-1)(qν-1) | 0,0 |
| (q3ν-qν)(q2ν+1)(q2ν+qν+1) | B1 | q3ν-qν,1 | q2ν,q2ν(qν+1) | q2ν+qν+1,q2ν(q2ν-1) | 0,0 | qν+1,q4ν(q2ν-1) |
| q3ν(q4ν-1)(q2ν-1) | B2 | 0,0 | q3ν-q2ν,q2ν+qν+1 | q3ν-q2ν-qν,q3ν-q2ν-qν | q3ν,q3ν | q3ν-qν,q6ν-q3ν |
The points of type C1 are on the secants only, while the points of type C2 are in the space "between" the secants. A tangent of type 1, "originating" in a point of the sub-geometry, is embedded in one, and one only, plane of type B0 and meets all the secants in that plane and not passing through that point of origin in a point of type C1, as well as having some points of type C2 in that plane. An exterior line of type 1 is also embedded in one, and one only, plane of type B0, and intersects all the secants in that plane in points lying "between" the points of the sub-geometry.
There are q3ν+1 planes through each secant: of these qν+1 are planes of type B0, so there are q3ν-qν of type B1. There are (q2ν+1)(q2ν+qν+1) secants, so the number of planes of type B1 is as shown. Supposing the sub-geometry points of this secant to be A,B,...,C, these planes then cut the planes of type B0 not passing through that secant, but passing through one of A,B,...,C, in tangents of type 1, and the remaining planes of type B0 in exterior lines of type 1.
Likewise, there are (q6ν+q3ν+1) planes through a point of the sub-geometry (say, A): of these, (q2ν+qν+1) are planes of the sub-geometry (i.e. of type B0) and a further (q3ν-qν) pass through each of the secants through u, leaving the given number of planes of type B2. Each of these planes meets the other planes through A in a tangent: so it must meet the planes of type B0 in a tangent of type 1, of which there are (q2ν+qν+1), which accounts for all the tangents of type 1 in a plane of type B2.
| q2ν+qν+1 | qν(q3ν-1)(qν+1) | 0 | q3ν(q2ν-1)(qν-1) | 0 | ||
|---|---|---|---|---|---|---|
| s | t1 | t2 | x1 | x2 | ||
| q2ν+qν+1 | C0 | qν+1,qν+1 | 1,q3ν-qν | 0,0 | 0,0 | 0,0 |
| qν(q3ν-1)(qν+1) | C1 | q3ν-qν,1 | q2ν,q2ν | 0,0 | q2ν+qν+1,q3ν-q2ν | 0,0 |
| q3ν(q2ν-1)(qν-1) | C2 | 0,0 | q3ν-q2ν, q2ν+qν+1 | 0,0 | q3ν-q2ν-qν,q3ν-q2ν-qν | 0,0 |
The complete design above is a BIBD with λ=1.
Since every line in the plane must meet every other such line in a point, the tangents containing point A (say) of the sub-geometry must meet all the secants not containing A in a point: in other words, the C1 points in the tangent comprise a transversal of the C1 points in the q2ν such secants. Likewise, the C1 points in an exterior line comprise a transversal of the C1 points in the q2ν+qν+1 secants.
| 1 | q2ν(qν+1) | q2ν(q2ν-1) | 0 | q4ν(q2ν-1) | ||
|---|---|---|---|---|---|---|
| s | t1 | t2 | x1 | x2 | ||
| qν+1 | C0 | qν+1,1 | 1,q2ν | 1,q3ν-q2ν | 0,0 | 0,0 |
| q3ν-qν | C11 | q3ν-qν,1 | 0,0 | 0,0 | 0,0 | 1,q3ν |
| q4ν | C12 | 0,0 | q2ν,qν+1 | 0,0 | 0,0 | qν,q3ν-qν |
| q4ν(q2ν-1) | C2 | 0,0 | q3ν-q2ν,1 | q3ν,qν | 0,0 | q3ν-qν,q3ν-qν |
The complete design above is a BIBD with λ=1.
In 3-space, there are q3ν+1 planes through a secant. Of these, only qν+1 pass through another secant, so the remainder have in addition just tangents and exterior lines.
There are no lines of type x1 in a plane of type B1. That this must be so may be seen as follows.
Suppose a plane α of type B0, containing an exterior line x of type 1. Firstly, no plane of type B1 containing a secant from α can contain x, obviously. So suppose a plane γ of type B1 containing a secant t (not in α), where t is contained by a plane β of type B0, distinct from α. The two planes α and β meet in a line, in particular in a secant, say s. Since s and t both lie in β, they must meet in a point, say A, belonging to the subgeometry. This must also belong to γ since it lies on t. If γ contains x, then it contains a point (of type C1) from each of the secants of α, so it contains two points from some secant of α, and therefore a whole secant from α, which is a contradiction.
The points of type C1 which occur in the secant clearly cannot appear in any tangents, so the class has been split in two, C11 and C12, to accommodate the tangents, where C11 comprises the points which lie on the secant. Clearly, no pair of points from C11 can occur together elsewhere in the geometry. This gives the structure of the exterior lines of type 2, the C1 points being split 1:qν between C11 and C12. The total number of C12 and C2 points is q6ν. The total number of tangents is (qν+1)q3ν, so the number of exterior lines (of type 2) is q4ν(q2ν-1).
This information is sufficient to determine successively: the occurrence of C12 points and their total number, hence the total number of C2 points, the occurrence of C0 points in a tangent of type 1, and so in a tangent of type 2, and the total number of such tangents, therefore the occurrence of C12 and C2 points in those tangents, and therefore in the exterior lines.
| 0 | q2ν+qν+1 | q3ν-q2ν-qν | q3ν | q3ν(q3ν-1) | ||
|---|---|---|---|---|---|---|
| s | t1 | t2 | x1 | x2 | ||
| 1 | C0 | 0,0 | 1,q2ν+qν+1 | 1,q3ν-q2ν-qν | 0,0 | 0,0 |
| q2ν(q2ν+qν+1) | C1 | 0,0 | q2ν,1 | 0,0 | q2ν+qν+1,qν | qν+1,q3ν-qν |
| (q2ν+qν+1)(q3ν-q2ν) | C21 | 0,0 | q3ν-q2ν,1 | 0,0 | 0,0 | q2ν,q3ν |
| q4ν(q2ν-qν-1) | C22 | 0,0 | 0,0 | q3ν,1 | q3ν-q2ν-qν,1 | q3ν-q2ν-qν,q3ν-1 |
The complete design above is a BIBD with λ=1.
The points of type C2 have been split into two sub-classes. Each point of type C1 and C2 can appear only once in a tangent, so if there are tangents of both types, the C2 points must be split, and each tangent type uses points from just one of the sub-classes.
The next step is to determine the number of type 1 tangents (the total number of tangents is (q3ν+1), so the total number of exterior lines is q6ν). Now, suppose the plane of type B2 is α and the C0 point in α is A. Through A there pass (q2ν+qν+1) planes of type B0: each of these meets α in a line, which must be a tangent of type 1. Conversely, each tangent of type 1 lies in one and one only plane of type B0 . Hence, the number of tangents of type 1 is (q2ν+qν+1), and that of type 2 tangents is (q3ν-q2ν-qν). This gives the number of C1, C21 and C22 points. The rest of the matrix can be obtained by solving a set of simultaneous equations.
Furthermore, there is the ability to drop points of two or more classes.
Last Updated on 06/05/2004
By D.H.Rees