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Search: id:A084553
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| A084553 |
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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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+0
3
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OFFSET
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1,2
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COMMENT
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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 A + A' where A runs through the matrices counted in A127503.
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LINKS
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Index entries for sequences related to binary matrices
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EXAMPLE
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The 5 X 5 matrix
2 -1 -1 -1 -1
-1 2 -1 -1 -1
-1 -1 2 -1 -1
-1 -1 -1 2 -1
-1 -1 -1 -1 2
with eigenvalues -2, 3, 3, 3, 3 is an example of one which is not positive semi-definite.
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CROSSREFS
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Cf. A084552, A085659, A080858, A083029.
Sequence in context: A139678 A152401 A009345 this_sequence A144164 A003091 A119501
Adjacent sequences: A084550 A084551 A084552 this_sequence A084554 A084555 A084556
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KEYWORD
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nonn
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AUTHOR
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N. J. A. Sloane (njas(AT)research.att.com), Jul 13 2003
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EXTENSIONS
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a(6)-a(9) from Max Alekseyev, Jan 16 2006
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