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A211443 Number of (n+1)X(n+1) -7..7 symmetric matrices with every 2X2 subblock having sum zero and one or three distinct values 1
91, 201, 397, 765, 1459, 2751, 5215, 9855, 18759, 35741, 68529, 131739, 254515, 493209, 959339, 1871379, 3660547, 7178105, 14104015, 27769453, 54754733, 108142387, 213809725, 423287363, 838643733, 1663340421, 3300898801 (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..210

FORMULA

Empirical: a(n) = 3*a(n-1) +9*a(n-2) -31*a(n-3) -30*a(n-4) +125*a(n-5) +48*a(n-6) -251*a(n-7) -46*a(n-8) +264*a(n-9) +40*a(n-10) -138*a(n-11) -28*a(n-12) +28*a(n-13) +8*a(n-14)

EXAMPLE

Some solutions for n=3

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

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

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

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

CROSSREFS

Sequence in context: A352133 A044423 A044804 * A305761 A339523 A260064

Adjacent sequences:  A211440 A211441 A211442 * A211444 A211445 A211446

KEYWORD

nonn

AUTHOR

R. H. Hardin Apr 11 2012

STATUS

approved

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Last modified May 27 16:48 EDT 2022. Contains 354110 sequences. (Running on oeis4.)