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A211471 Number of (n+1)X(n+1) -8..8 symmetric matrices with every 2X2 subblock having sum zero and three or four distinct values 1
120, 856, 6040, 42492, 297572, 2076768, 14447284, 100224428, 693541160, 4788481556, 32994512924, 226924908072, 1558056553740, 10680681136996, 73109657722272, 499746329784652, 3411594843779188, 23261004734670688 (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

R. H. Hardin, Table of n, a(n) for n = 1..30

EXAMPLE

Some solutions for n=3

.-7..4.-3..6....8.-1..6..0....4.-6..4.-5....6..1..2.-4....1..2..2.-4

..4.-1..0.-3...-1.-6..1.-7...-6..8.-6..7....1.-8..5.-3....2.-5..1..1

.-3..0..1..2....6..1..4..2....4.-6..4.-5....2..5.-2..0....2..1..3.-5

..6.-3..2.-5....0.-7..2.-8...-5..7.-5..6...-4.-3..0..2...-4..1.-5..7

CROSSREFS

Sequence in context: A235551 A300664 A029573 * A250519 A250512 A011245

Adjacent sequences:  A211468 A211469 A211470 * A211472 A211473 A211474

KEYWORD

nonn

AUTHOR

R. H. Hardin Apr 12 2012

STATUS

approved

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Last modified September 23 11:53 EDT 2020. Contains 337310 sequences. (Running on oeis4.)