OFFSET
1,1
COMMENTS
Symmetry and 2 X 2 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).
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..210
FORMULA
Empirical: a(n) = 2*a(n-1) + 3*a(n-2) - 6*a(n-3) - 2*a(n-4) + 4*a(n-5).
Empirical g.f.: x*(9 + x - 26*x^2 + 20*x^4) / ((1 - x)*(1 + x)*(1 - 2*x)*(1 - 2*x^2)). - Colin Barker, Jul 15 2018
EXAMPLE
Some solutions for n=3:
..2.-1..1.-1....2..0..1.-2....2.-1..0.-1....0..1..1..0...-2..1.-2..1
.-1..0..0..0....0.-2..1..0...-1..0..1..0....1.-2..0.-1....1..0..1..0
..1..0..0..0....1..1..0.-1....0..1.-2..1....1..0..2.-1...-2..1.-2..1
.-1..0..0..0...-2..0.-1..2...-1..0..1..0....0.-1.-1..0....1..0..1..0
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Apr 02 2012
STATUS
approved