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%I #4 Apr 11 2012 07:19:10
%S 91,201,397,765,1459,2751,5215,9855,18759,35741,68529,131739,254515,
%T 493209,959339,1871379,3660547,7178105,14104015,27769453,54754733,
%U 108142387,213809725,423287363,838643733,1663340421,3300898801
%N Number of (n+1)X(n+1) -7..7 symmetric matrices with every 2X2 subblock having sum zero and one or three distinct values
%C Symmetry and 2X2 block sums zero implies that the diagonal x(i,i) are equal modulo 2 and x(i,j)=(x(i,i)+x(j,j))/2*(-1)^(i-j)
%H R. H. Hardin, <a href="/A211443/b211443.txt">Table of n, a(n) for n = 1..210</a>
%F Empirical: a(n) = 3*a(n-1) +9*a(n-2) -31*a(n-3) -30*a(n-4) +125*a(n-5) +48*a(n-6) -251*a(n-7) -46*a(n-8) +264*a(n-9) +40*a(n-10) -138*a(n-11) -28*a(n-12) +28*a(n-13) +8*a(n-14)
%e Some solutions for n=3
%e .-5..2.-5..2...-7..3.-7..2....6..0..6.-3....3.-5..3..1...-6..0.-3..6
%e ..2..1..2..1....3..1..3..2....0.-6..0.-3...-5..7.-5..1....0..6.-3..0
%e .-5..2.-5..2...-7..3.-7..2....6..0..6.-3....3.-5..3..1...-3.-3..0..3
%e ..2..1..2..1....2..2..2..3...-3.-3.-3..0....1..1..1.-5....6..0..3.-6
%K nonn
%O 1,1
%A _R. H. Hardin_ Apr 11 2012
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