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 A211711 Number of (n+1)X(n+1) -11..11 symmetric matrices with every 2X2 subblock having sum zero and three distinct values 1
 230, 516, 1030, 1908, 3528, 6312, 11390, 20316, 36520, 65646, 118702, 216006, 394802, 728492, 1348550, 2522796, 4730304, 8960110, 16997998, 32539860, 62348810, 120381720, 232534604, 451979570, 878630354, 1716513328, 3353186654 (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) = 6*a(n-1) +3*a(n-2) -83*a(n-3) +92*a(n-4) +443*a(n-5) -868*a(n-6) -1073*a(n-7) +3394*a(n-8) +748*a(n-9) -7085*a(n-10) +1842*a(n-11) +8231*a(n-12) -4528*a(n-13) -5075*a(n-14) +3936*a(n-15) +1424*a(n-16) -1511*a(n-17) -121*a(n-18) +244*a(n-19) -6*a(n-20) -12*a(n-21) EXAMPLE Some solutions for n=3 ..1..2.-5..2...-8..2.-8..3...11.-7..3.-5....7.-8..7.-8....5..1..1..1 ..2.-5..8.-5....2..4..2..3...-7..3..1..1...-8..9.-8..9....1.-7..5.-7 .-5..8-11..8...-8..2.-8..3....3..1.-5..3....7.-8..7.-8....1..5.-3..5 ..2.-5..8.-5....3..3..3..2...-5..1..3.-1...-8..9.-8..9....1.-7..5.-7 CROSSREFS Sequence in context: A171666 A140077 A215217 * A211716 A194490 A051183 Adjacent sequences:  A211708 A211709 A211710 * A211712 A211713 A211714 KEYWORD nonn AUTHOR R. H. Hardin Apr 20 2012 STATUS approved

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