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A211554 Number of (n+1)X(n+1) -9..9 symmetric matrices with every 2X2 subblock having sum zero and three or four distinct values 1
150, 1194, 9390, 73506, 572242, 4435846, 34254414, 263666834, 2023872594, 15497836998, 118429480822, 903378800274, 6880209099890, 52328684259574, 397516660098150, 3016531736769506, 22869029384939666, 173227696174568614 (list; graph; refs; listen; history; text; internal format)
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
.-2.-3..0.-2...-4..5.-6..2....5..2..1.-5...-2..5.-2..1....9.-6..9.-2
.-3..8.-5..7....5.-6..7.-3....2.-9..6.-2....5.-8..5.-4...-6..3.-6.-1
..0.-5..2.-4...-6..7.-8..4....1..6.-3.-1...-2..5.-2..1....9.-6..9.-2
.-2..7.-4..6....2.-3..4..0...-5.-2.-1..5....1.-4..1..0...-2.-1.-2.-5
CROSSREFS
Sequence in context: A054558 A250544 A251801 * A073614 A191547 A184673
KEYWORD
nonn
AUTHOR
R. H. Hardin Apr 15 2012
STATUS
approved

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Last modified April 21 00:02 EDT 2024. Contains 371850 sequences. (Running on oeis4.)