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A211469 Number of (n+1)X(n+1) -8..8 symmetric matrices with every 2X2 subblock having sum zero and two or three distinct values 1
144, 380, 856, 1808, 3706, 7454, 14786, 29300, 57442, 113394, 221676, 437914, 857230, 1698058, 3335170, 6630316, 13078566, 26101324, 51724674, 103628922, 206316160, 414882652, 829705256, 1674253282, 3362428628, 6806756646 (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) +20*a(n-2) -64*a(n-3) -172*a(n-4) +598*a(n-5) +826*a(n-6) -3213*a(n-7) -2390*a(n-8) +10954*a(n-9) +4103*a(n-10) -24660*a(n-11) -3463*a(n-12) +36968*a(n-13) -524*a(n-14) -36330*a(n-15) +4151*a(n-16) +22474*a(n-17) -3786*a(n-18) -8144*a(n-19) +1410*a(n-20) +1540*a(n-21) -180*a(n-22) -120*a(n-23)

EXAMPLE

Some solutions for n=3

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

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

..1.-2..1..1....0..0.-5..0....1..2..0..2...-4..2..0..2....1..1..1.-2

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

CROSSREFS

Sequence in context: A189988 A232892 A034285 * A248551 A178972 A250787

Adjacent sequences:  A211466 A211467 A211468 * A211470 A211471 A211472

KEYWORD

nonn

AUTHOR

R. H. Hardin Apr 12 2012

STATUS

approved

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Last modified June 15 15:16 EDT 2021. Contains 345049 sequences. (Running on oeis4.)