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A211447
Number of (n+1)X(n+1) -7..7 symmetric matrices with every 2X2 subblock having sum zero and one, three or four distinct values
1
91, 561, 3397, 20495, 123133, 738307, 4420961, 26449067, 158114037, 944443595, 5635861085, 33592371427, 199953831841, 1188364420259, 7050730029429, 41757312808091, 246836304140925, 1456269364652499, 8574736487657897
OFFSET
1,1
COMMENTS
Symmetry and 2X2 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
EXAMPLE
Some solutions for n=3
.-3.-2.-4..5....5.-1..6..1...-1..2..3..1....3.-4..2.-1....7..0..1.-7
.-2..7.-1..0...-1.-3.-2.-5....2.-3.-2.-2...-4..5.-3..2....0.-7..6..0
.-4.-1.-5..6....6.-2..7..0....3.-2..7.-3....2.-3..1..0....1..6.-5.-1
..5..0..6.-7....1.-5..0.-7....1.-2.-3.-1...-1..2..0.-1...-7..0.-1..7
CROSSREFS
Sequence in context: A373526 A373521 A180942 * A188360 A165220 A020218
KEYWORD
nonn
AUTHOR
R. H. Hardin Apr 11 2012
STATUS
approved