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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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2
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OFFSET
| 1,2
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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 A + A' where A runs through the matrices counted in A127503.
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LINKS
| Index entries for sequences related to binary matrices
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EXAMPLE
| 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
| Cf. A084552, A085659, A080858, A083029.
Sequence in context: A139678 A152401 A009345 * A144164 A003091 A119501
Adjacent sequences: A084550 A084551 A084552 * A084554 A084555 A084556
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KEYWORD
| nonn
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), Jul 13 2003
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EXTENSIONS
| a(6)-a(9) from Max Alekseyev, Jan 16 2006
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