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..192
FORMULA
Empirical: a(n) = 2*a(n-1) - a(n-3).
Empirical g.f.: x*(31 - 19*x - 25*x^2) / ((1 - x)*(1 - x - x^2)). - Colin Barker, Jul 19 2018
EXAMPLE
Some solutions for n=3:
..9.-3..3.-9....3.-3..3.-3...-1.-1.-1..1....7.-7..7.-7....2.-2..2.-2
.-3.-3..3..3...-3..3.-3..3...-1..3.-1..1...-7..7.-7..7...-2..2.-2..2
..3..3.-3.-3....3.-3..3.-3...-1.-1.-1..1....7.-7..7.-7....2.-2..2.-2
.-9..3.-3..9...-3..3.-3..3....1..1..1.-1...-7..7.-7..7...-2..2.-2..2
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Apr 15 2012
STATUS
approved