
COMMENTS

Each solution corresponds to a Hadamard matrix of quaternion type. That is, if H = [[A, B, C, D], [B, A, D, C], [C, D, A, B], [D, C, B, A]], where A,B,C, and D are circulant matrices formed from a,b,c and d, respectively, then H is Hadamard.
Since a,b,c and d are even, their discrete Fourier transforms are realvalued.
16 is a divisor of a(n), for all n. If (a,b,c,d) is a solution, then each of the 16 tuples ((+)a, (+)b, (+)c, (+)d) is also a solution.
It appears that a(2n) > a(2n1).
