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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: A281246 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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