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A211549 Number of (n+1)X(n+1) -9..9 symmetric matrices with every 2X2 subblock having sum zero and one or two distinct values 1
31, 43, 61, 91, 139, 217, 343, 547, 877, 1411, 2275, 3673, 5935, 9595, 15517, 25099, 40603, 65689, 106279, 171955, 278221, 450163, 728371, 1178521, 1906879, 3085387, 4992253, 8077627, 13069867, 21147481, 34217335, 55364803, 89582125, 144946915 (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..192

FORMULA

Empirical: a(n) = 2*a(n-1) -a(n-3)

EXAMPLE

Some solutions for n=3

..9.-3..3.-9....3.-3..3.-3...-1.-1.-1..1....7.-7..7.-7....2.-2..2.-2

.-3.-3..3..3...-3..3.-3..3...-1..3.-1..1...-7..7.-7..7...-2..2.-2..2

..3..3.-3.-3....3.-3..3.-3...-1.-1.-1..1....7.-7..7.-7....2.-2..2.-2

.-9..3.-3..9...-3..3.-3..3....1..1..1.-1...-7..7.-7..7...-2..2.-2..2

CROSSREFS

Sequence in context: A089713 A067467 A069455 * A118637 A096163 A139883

Adjacent sequences:  A211546 A211547 A211548 * A211550 A211551 A211552

KEYWORD

nonn

AUTHOR

R. H. Hardin Apr 15 2012

STATUS

approved

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Last modified December 17 18:04 EST 2014. Contains 252034 sequences.