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 A211715 Number of (n+1) X (n+1) -11..11 symmetric matrices with every 2 X 2 subblock having sum zero and two or four distinct values. 1
 34, 58, 106, 202, 394, 778, 1546, 3082, 6154, 12298, 24586, 49162, 98314, 196618, 393226, 786442, 1572874, 3145738, 6291466, 12582922, 25165834, 50331658, 100663306, 201326602, 402653194, 805306378, 1610612746, 3221225482 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Symmetry and 2 X 2 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) = 3*a(n-1) - 2*a(n-2). Conjectures from Colin Barker, Jul 19 2018: (Start) G.f.: 2*x*(17 - 22*x) / ((1 - x)*(1 - 2*x)). a(n) = 10 + 3*2^(2+n). (End) EXAMPLE Some solutions for n=3: ..9.-3..3.-9....3..3.-3..3...-6..2.-6..2...-1..1..1.-1...-8..8.-8..8 .-3.-3..3..3....3.-9..9.-9....2..2..2..2....1.-1.-1..1....8.-8..8.-8 ..3..3.-3.-3...-3..9.-9..9...-6..2.-6..2....1.-1..3.-3...-8..8.-8..8 .-9..3.-3..9....3.-9..9.-9....2..2..2..2...-1..1.-3..3....8.-8..8.-8 CROSSREFS Sequence in context: A224896 A103558 A103686 * A108610 A261314 A119454 Adjacent sequences: A211712 A211713 A211714 * A211716 A211717 A211718 KEYWORD nonn AUTHOR R. H. Hardin, Apr 20 2012 STATUS approved

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Last modified October 2 14:37 EDT 2023. Contains 365837 sequences. (Running on oeis4.)