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A139678
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Number of n X n symmetric binary matrices with no row sum greater than 2.
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1
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1, 2, 8, 45, 315, 2634, 25518, 280257, 3434595, 46400310, 684374076, 10933866027, 187983528813, 3458845917990, 67787903801790, 1409293876400019, 30968525550983913, 717023025711440082, 17442766619178969600
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OFFSET
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0,2
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LINKS
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Nathaniel Johnston, Table of n, a(n) for n = 0..200
Problem discussed on the Art of Problem Solving forum
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FORMULA
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E.g.f.: exp( (6x + x^2 + x^2)/(4(1 - x)) ) / sqrt(1 - x). - Joel B. Lewis, Apr 17 2011
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MAPLE
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n:=18: G:=taylor((1/sqrt(1-x))*exp((6*x + x^2 + x^3)/(4 - 4*x)), x=0, n+1): seq(coeff(G, x, m)*m!, m=0..n); # Nathaniel Johnston, Apr 19 2011
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CROSSREFS
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Sequence in context: A002833 A139015 A197996 * A152401 A009345 A084553
Adjacent sequences: A139675 A139676 A139677 * A139679 A139680 A139681
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KEYWORD
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nonn
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AUTHOR
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R. H. Hardin, Jun 13 2008
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STATUS
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approved
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