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COMMENTS
| A conference matrix of order n is an n X n {-1,0,+1} matrix A such that A A' = (n-1)I.
If n == 2 (mod 4) then a necessary condition is that n-1 is a sum of 2 squares. It is conjectured that this condition is also sufficient. If n == 2 mod 4 and n-1 is a prime or prime power the condition is automatically satisfied.
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REFERENCES
| V. Belevitch, Conference matrices and Hadamard matrices, Ann. Soc. Scientifique Bruxelles, 82 (I) (1968), 13-32.
CRC Handbook of Combinatorial Designs, 1996, Chapter 52.
F. J. MacWilliams and N. J. A. Sloane, The Theory of Error-Correcting Codes, Elsevier-North Holland, 1978, p. 56.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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