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A211550 Number of (n+1)X(n+1) -9..9 symmetric matrices with every 2X2 subblock having sum zero and three distinct values 1
150, 330, 640, 1190, 2172, 3922, 7052, 12734, 22944, 41816, 76046, 140318, 258302, 482748, 900018, 1702150, 3211388, 6136202, 11697780, 22541648, 43345282, 84091266, 162828522, 317556046, 618284124, 1210700090, 2367405022, 4650241676 (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) = 5*a(n-1) +6*a(n-2) -66*a(n-3) +32*a(n-4) +343*a(n-5) -396*a(n-6) -874*a(n-7) +1475*a(n-8) +1066*a(n-9) -2702*a(n-10) -373*a(n-11) +2577*a(n-12) -367*a(n-13) -1195*a(n-14) +320*a(n-15) +210*a(n-16) -60*a(n-17)

EXAMPLE

Some solutions for n=3

..4.-2..4..0....6.-3..0.-6....1..3..1.-2....4..2..4.-3....9.-2..9.-2

.-2..0.-2.-2...-3..0..3..3....3.-7..3.-2....2.-8..2.-3...-2.-5.-2.-5

..4.-2..4..0....0..3.-6..0....1..3..1.-2....4..2..4.-3....9.-2..9.-2

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

CROSSREFS

Sequence in context: A063829 A291959 A210643 * A212465 A273322 A206066

Adjacent sequences:  A211547 A211548 A211549 * A211551 A211552 A211553

KEYWORD

nonn

AUTHOR

R. H. Hardin Apr 15 2012

STATUS

approved

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Last modified September 19 15:17 EDT 2021. Contains 347564 sequences. (Running on oeis4.)