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A211322
Number of (n+1) X (n+1) -3..3 symmetric matrices with every 2 X 2 subblock having sum zero and one or two distinct values.
1
11, 15, 21, 31, 47, 73, 115, 183, 293, 471, 759, 1225, 1979, 3199, 5173, 8367, 13535, 21897, 35427, 57319, 92741, 150055, 242791, 392841, 635627, 1028463, 1664085, 2692543, 4356623, 7049161, 11405779, 18454935, 29860709, 48315639, 78176343
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
FORMULA
Empirical: a(n) = 2*a(n-1) - a(n-3).
Conjectures from Colin Barker, Jul 16 2018: (Start)
G.f.: x*(11 - 7*x - 9*x^2) / ((1 - x)*(1 - x - x^2)).
a(n) = 5 + (2^(1-n)*((1-sqrt(5))^n*(-2+sqrt(5)) + (1+sqrt(5))^n*(2+sqrt(5)))) / sqrt(5).
(End)
EXAMPLE
Some solutions for n=3:
.-1.-1.-1..1...-1..1.-1.-1....0..0..0..0...-3..1.-3..1....3.-3..3.-3
.-1..3.-1..1....1.-1..1..1....0..0..0..0....1..1..1..1...-3..3.-3..3
.-1.-1.-1..1...-1..1.-1.-1....0..0..0..0...-3..1.-3..1....3.-3..3.-3
..1..1..1.-1...-1..1.-1..3....0..0..0..0....1..1..1..1...-3..3.-3..3
CROSSREFS
Sequence in context: A237278 A265114 A213179 * A087682 A104628 A045564
KEYWORD
nonn
AUTHOR
R. H. Hardin, Apr 07 2012
STATUS
approved