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) - a(n-3).
Empirical g.f.: x*(25 - 17*x - 21*x^2) / ((1 - x)*(1 - x - x^2)). - Colin Barker, Jul 18 2018
EXAMPLE
Some solutions for n=3:
.-3..1.-1..3...-5..5.-5..5...-3..3.-3..3....2..2..2..2...-2.-2.-2.-2
..1..1.-1.-1....5.-5..5.-5....3.-3..3.-3....2.-6..2.-6...-2..6.-2..6
.-1.-1..1..1...-5..5.-5..5...-3..3.-3..3....2..2..2..2...-2.-2.-2.-2
..3.-1..1.-3....5.-5..5.-5....3.-3..3.-3....2.-6..2.-6...-2..6.-2..6
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Apr 12 2012
STATUS
approved