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) = 3*a(n-1) + 5*a(n-2) - 20*a(n-3) - 5*a(n-4) + 45*a(n-5) - 5*a(n-6) - 40*a(n-7) + 6*a(n-8) + 12*a(n-9).
Empirical g.f.: x*(29 - 22*x - 201*x^2 + 125*x^3 + 482*x^4 - 225*x^5 - 480*x^6 + 144*x^7 + 176*x^8) / ((1 - x)*(1 + x)*(1 - 2*x)*(1 - x - x^2)*(1 - 2*x^2)*(1 - 3*x^2)). - Colin Barker, Jul 18 2018
EXAMPLE
Some solutions for n=3:
.-1..2.-1..2...-4..2..0..2....2.-1..2.-3....0..2..0..0...-2..3.-2..3
..2.-3..2.-3....2..0.-2..0...-1..0.-1..2....2.-4..2.-2....3.-4..3.-4
.-1..2.-1..2....0.-2..4.-2....2.-1..2.-3....0..2..0..0...-2..3.-2..3
..2.-3..2.-3....2..0.-2..0...-3..2.-3..4....0.-2..0..0....3.-4..3.-4
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Apr 13 2012
STATUS
approved