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A027832
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Number of symmetric {-1, +1} matrices of order n with nonnegative row and column sums.
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0
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1, 5, 14, 315, 2634, 301262, 8035168, 4451407563, 392447922178, 1028823851939030, 306635655405986312, 3743721825782942609558, 3832881704712490758507152, 215313862348697780762374273824, 766493362781127793638943080523776, 196222469681558506008588371784165178651, 2452358136558134202145977229121349160510450
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,2
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REFERENCES
| I. Anderson, Combinatorics of Finite Sets, Oxford 1987, Ch. 3.1.
C. Greene and D. J. Kleitman, Strong versions of Sperner's Theorem, J. Combin. Theory, A 20 (1976) 80-88.
Torsten Sillke and Achim Flammenkamp, unpublished.
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EXAMPLE
| n=2: A(2) = 5 from ++/++, ++/+-, -+/++, -+/+-, +-/-+
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CROSSREFS
| Sequence in context: A027304 A070135 A067857 * A128946 A156219 A000331
Adjacent sequences: A027829 A027830 A027831 * A027833 A027834 A027835
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KEYWORD
| nonn,nice
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AUTHOR
| Torsten Sillke (sillke(AT)Mathematik.Uni-Bielefeld.DE)
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