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 A085658 Number of n X n symmetric positive semi-definite matrices with 2's on the main diagonal and 1's and 0's elsewhere. 6
 1, 2, 8, 64, 924, 21737, 749684, 33568376 (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 semi-definite) is 2^C(n,2). This gives the number of different values of M + M' where M runs through the matrices counted in A038379. - Max Alekseyev, Nov 11 2006 LINKS EXAMPLE The matrix 2 0 0 0 1 0 2 0 1 1 0 0 2 1 1 0 1 1 2 0 1 1 1 0 2 is one of the 100 5 X 5 matrices which are not positive semi-definite. Its eigenvalues are approximately [2., -0.135779205069857, 4.135779205069857, 1.337846553138044, 2.662153446861956] CROSSREFS Cf. A085658, A080858, A083029. Sequence in context: A134956 A011803 A007625 * A153541 A153569 A153532 Adjacent sequences:  A085655 A085656 A085657 * A085659 A085660 A085661 KEYWORD nonn,more AUTHOR N. J. A. Sloane, Jul 12 2003 EXTENSIONS 3 more terms from Max Alekseyev, Nov 08 2006 STATUS approved

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Last modified June 28 19:10 EDT 2022. Contains 354907 sequences. (Running on oeis4.)