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A211815 Number of (n+1)X(n+1) -10..10 symmetric matrices with every 2X2 subblock having sum zero and one or three distinct values 1
189, 435, 881, 1701, 3247, 6081, 11421, 21369, 40139, 75675, 143095, 272447, 519721, 999049, 1922557, 3726521, 7226935, 14102879, 27524803, 53995609, 105915029, 208603523, 410770735, 811441587, 1602540065, 3172707173, 6279776411 (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) +8*a(n-2) -75*a(n-3) +16*a(n-4) +465*a(n-5) -403*a(n-6) -1541*a(n-7) +1945*a(n-8) +2941*a(n-9) -4737*a(n-10) -3219*a(n-11) +6630*a(n-12) +1827*a(n-13) -5437*a(n-14) -299*a(n-15) +2516*a(n-16) -186*a(n-17) -591*a(n-18) +94*a(n-19) +54*a(n-20) -12*a(n-21)

EXAMPLE

Some solutions for n=3

..5.-3..5.-1....2.-1..1.-2....4..2..3..2...-5..4.-3..2...-2..3.-2..3

.-3..1.-3.-1...-1..0..0..1....2.-8..3.-8....4.-3..2.-1....3.-4..3.-4

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

.-1.-1.-1.-3...-2..1.-1..2....2.-8..3.-8....2.-1..0..1....3.-4..3.-4

CROSSREFS

Sequence in context: A320709 A224674 A076759 * A347390 A292173 A231394

Adjacent sequences:  A211812 A211813 A211814 * A211816 A211817 A211818

KEYWORD

nonn

AUTHOR

R. H. Hardin Apr 21 2012

STATUS

approved

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Last modified October 20 03:04 EDT 2021. Contains 348099 sequences. (Running on oeis4.)