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A085658
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Number of n X n symmetric positive semi-definite matrices with 2's on the main diagonal and 1's and 0's elsewhere.
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6
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OFFSET
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1,2
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COMMENTS
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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
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LINKS
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Table of n, a(n) for n=1..8.
Index entries for sequences related to binary matrices
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EXAMPLE
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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]
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CROSSREFS
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Cf. A085658, A080858, A083029.
Sequence in context: A134956 A011803 A007625 * A153541 A153569 A153532
Adjacent sequences: A085655 A085656 A085657 * A085659 A085660 A085661
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KEYWORD
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nonn,more
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AUTHOR
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N. J. A. Sloane, Jul 12 2003
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EXTENSIONS
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3 more terms from Max Alekseyev, Nov 08 2006
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STATUS
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approved
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