Hadamard Matrices
It has been a common misapprehension over the last seven or eight years that the number of pairwise non-isomorphic 2-(23,11,5) designs is 1102. This notion was first put forward by H. Kimura [1] who found a Hadamard matrix of order 24 that had been missed by Ito et al [2]. It is a figure that appears even today in the new CRC Handbook of Combinatorial Designs. In my recent paper on Hadamard matrices of orders 24 and 28 [3], I also repeated this erroneous figure. Had I bothered to check the numbers of Hadamard designs that I had found from the 60 Hadamard matrices of order 24, I would have announced that the correct number of pairwise non-isomorphic 2-(23,11,5) designs is 1106. This number is implicit in the Table on pp 192-196 appearing in my paper cited above, and indeed all 1106 designs are contained in the gzipped file 23-11-5.gz.Some people may be interested in having the electronic copies of the Hadamard matrices of orders up to 28. Since they are unique for orders 2, 4, 8 and 12, I only list those of orders 16, 20, 24 and 28. These can be found in the files Hadamard.16, Hadamard.20, Hadamard.24, Hadamard.28. These are files of zero-one matrices of orders 15, 19, 23 and 27, respectively which are obtained from the normalised Hadamard matrix of order 4m by deleting the first row and column, and replacing the -1's by 0's.
It is a well-known fact that regular two-graphs on 36 vertices
correspond to regular symmetric Hadamard matrices with constant
diagonal. In a recent investigation into such regular two-graphs [4],
136 new ones were found, giving a total of
227. Not all these non-isomorphic regular two-graphs were
non-isomorphic as Hadamard matrices. In fact the number of pairwise
non-isomorphic Hadamard matrices thus found is 180. These Hadamard
matrices were examined for Hadamard designs (symmetric 2-(35,17,8)
designs) and I discovered that they yielded 101,863 in number (NOT
108,131 as I wrote in the above paper!!). I include the 180
Hadamard matrices in the file
Hadamard.36,
where also included is the order of
the automorphism group of the Hadamard matrix as well as the number
of non-isomorphic descendants that arise from each. In common with
the listing of regular two-graphs that appear in
Regular Two-Graphs, the notation [x,y] means x descendants with automorphism group of order y.
When we had nothing better to do Dick Turyn and I had a look at the
numbers of Hadamard matrices of various orders of the Goethals-Seidel
type. The numbers of such pairwise non-isomorphic matrices grows
rapidly with the order, as one might expect. When the order is 36 we
found 24, none of which appears among the 180 referred to above.
For completeness they are listed here in (0,1) form
G-S matrices of
order 36 , along with the order of their automorphism group,
the numbers of descendants and the orders of their automorphism groups.
To obtain the Hadamard matrix, replace the zeros with -1's.
[1] H. Kimura (New Hadamard matrix of order 24, Graphs Combin. 2 (1986) 247-257)
[2] Ito et al, Classification of 3-(24,12,5) designs and 24-dimensional Hadamard matrices, J. Comb. Theory Ser. A 27 (1979) 289-306.
[3] E Spence, Classification of Hadamard Matrices of Orders 24 and 28, Discrete Math., 140 (1995) 185-243.
[4] E. Spence, Regular two-graphs on 36 vertices, Lin. Alg. Appl.,
226-228 (1995) 459-497.