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 A211494 Number of (n+1)X(n+1) -4..4 symmetric matrices with every 2X2 subblock having sum zero and one, two or three distinct values 1
 41, 105, 259, 619, 1455, 3389, 7821, 18047, 41509, 95753, 220953, 511805, 1187991, 2768195, 6468085, 15166639, 35663339, 84116703, 198906647, 471540601, 1120285205, 2667019663, 6360447181, 15193138613, 36342090585, 87038546365 (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) +2*a(n-2) -73*a(n-3) +72*a(n-4) +364*a(n-5) -568*a(n-6) -954*a(n-7) +1943*a(n-8) +1391*a(n-9) -3685*a(n-10) -1073*a(n-11) +4101*a(n-12) +326*a(n-13) -2657*a(n-14) +54*a(n-15) +953*a(n-16) -47*a(n-17) -172*a(n-18) +6*a(n-19) +12*a(n-20) EXAMPLE Some solutions for n=3 .-1..0.-1..2....1..1..1..0...-2..0.-2..1....4.-2..2..0....2.-1..0.-1 ..0..1..0.-1....1.-3..1.-2....0..2..0..1...-2..0..0.-2...-1..0..1..0 .-1..0.-1..2....1..1..1..0...-2..0.-2..1....2..0..0..2....0..1.-2..1 ..2.-1..2.-3....0.-2..0.-1....1..1..1..0....0.-2..2.-4...-1..0..1..0 CROSSREFS Sequence in context: A115163 A044228 A044609 * A142054 A279213 A070270 Adjacent sequences:  A211491 A211492 A211493 * A211495 A211496 A211497 KEYWORD nonn AUTHOR R. H. Hardin Apr 13 2012 STATUS approved

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Last modified January 26 09:35 EST 2022. Contains 350598 sequences. (Running on oeis4.)