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 A086900 Number of real n X n symmetric (0,1) matrices with positive determinant. 5
 1, 1, 5, 338, 14186, 526876, 52658844 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS EXAMPLE For n = 2 the only example is the identity matrix. MATHEMATICA triamat[li_List] := Block[{len=Sqrt[8Length[li]+1]/2-1/2}, If[IntegerQ[len], Part[li, # ]& /@ Table[If[j>i, j(j-1)/2+i, i(i-1)/2+j], {i, len}, {j, len}], li]]; Table[it=triamat/@ IntegerDigits[Range[0, -1+2^(n(n+1)/2)], 2, n(n+1)/2]; Count[it, (q_)?MatrixQ/; (Det[q]>0)], {n, 5}] CROSSREFS Cf. A086899, A118996. Sequence in context: A316128 A317434 A029495 * A200002 A265921 A203684 Adjacent sequences:  A086897 A086898 A086899 * A086901 A086902 A086903 KEYWORD nonn,more AUTHOR Wouter Meeussen Aug 23, 2003. EXTENSIONS a(6) and a(7) from Giovanni Resta, May 08 2006 STATUS approved

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Last modified December 13 22:55 EST 2019. Contains 329974 sequences. (Running on oeis4.)