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 A084552 Number of n X n symmetric positive definite matrices with 2's on the main diagonal and -1's and 0's elsewhere. 2
 1, 2, 7, 38, 286, 2686, 28512, 312572, 3337588, 40963216 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Of course the total number of symmetric matrices of this type (not necessarily positive definite) is 2^C(n,2). This gives the number of different values of M + M' where M runs through the matrices counted in A127502. LINKS EXAMPLE The singular matrix 2 -1 -1 -1 2 -1 -1 -1 2 is the only 3 X 3 matrix of this type which is not positive definite. CROSSREFS Cf. A084553, A085659, A080858, A083029. Sequence in context: A088792 A114160 A145159 * A094664 A001858 A233335 Adjacent sequences:  A084549 A084550 A084551 * A084553 A084554 A084555 KEYWORD nonn AUTHOR N. J. A. Sloane, Jul 13 2003 EXTENSIONS a(6)-a(10) from Max Alekseyev, Jan 16 2006 STATUS approved

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Last modified October 17 08:38 EDT 2018. Contains 316276 sequences. (Running on oeis4.)