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 A211257 Number of (n+1)X(n+1) -6..6 symmetric matrices with every 2X2 subblock having sum zero and one, two or three distinct values 1
 85, 231, 547, 1283, 2901, 6595, 14775, 33409, 75157, 170723, 387687, 888293, 2039449, 4718583, 10949039, 25566565, 59877913, 140930679, 332585711, 787830185, 1870341077, 4452666039, 10618825855, 25375634725, 60720707925 (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) +6*a(n-2) -97*a(n-3) +62*a(n-4) +668*a(n-5) -852*a(n-6) -2556*a(n-7) +4359*a(n-8) +5935*a(n-9) -12593*a(n-10) -8545*a(n-11) +22727*a(n-12) +7400*a(n-13) -26431*a(n-14) -3396*a(n-15) +19783*a(n-16) +389*a(n-17) -9298*a(n-18) +302*a(n-19) +2606*a(n-20) -118*a(n-21) -392*a(n-22) +12*a(n-23) +24*a(n-24) EXAMPLE Some solutions for n=3 ..3.-2..1.-2...-4..2.-2..1....4..0..0..0...-2..1.-1..2....3.-1..2.-1 .-2..1..0..1....2..0..0..1....0.-4..4.-4....1..0..0.-1...-1.-1..0.-1 ..1..0.-1..0...-2..0..0.-1....0..4.-4..4...-1..0..0..1....2..0..1..0 .-2..1..0..1....1..1.-1..2....0.-4..4.-4....2.-1..1.-2...-1.-1..0.-1 CROSSREFS Sequence in context: A044798 A260100 A072289 * A027524 A043340 A045129 Adjacent sequences:  A211254 A211255 A211256 * A211258 A211259 A211260 KEYWORD nonn AUTHOR R. H. Hardin Apr 06 2012 STATUS approved

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Last modified December 12 20:12 EST 2019. Contains 329961 sequences. (Running on oeis4.)