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A087488
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Number of n X n (-1,1)-matrices with all eigenvalues >= 0.
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1
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OFFSET
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0,3
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LINKS
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EXAMPLE
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For n = 2 the six matrices are (+ means +1, - means -1):
++ +- -- -+ +- ++
-- +- ++ -+ -+ ++
with eigenvalues
00 00 00 00 20 20 respectively.
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MATHEMATICA
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a[n_] := Select[Partition[#, n] & /@ Tuples[{-1, 1}, {n^2}], AllTrue[ Eigenvalues[#], NonNegative]&] // Length; a[0] = 1;
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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Frederique Oggier (frederique.oggier(AT)epfl.ch) and N. J. A. Sloane, Oct 24 2003
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EXTENSIONS
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STATUS
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approved
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