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A211774
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Number of rooted 2-regular labeled graphs on n nodes.
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1
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0, 0, 0, 3, 12, 60, 420, 3255, 28056, 270144, 2868840, 33293205, 419329020, 5697423732, 83069039508, 1293734268645, 21436030749840, 376516868504160, 6988441065717744, 136675039085498691, 2809247116432575420, 60543293881318183740, 1365186080156105513460
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OFFSET
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0,4
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LINKS
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FORMULA
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E.g.f.: x*A'(x) where A(x) = exp(-x/2-x^2/4)/sqrt(1-x) is the e.g.f. for A001205.
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MAPLE
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egf:= x *diff(exp(-x/2-x^2/4)/sqrt(1-x), x):
a:= n-> n! * coeff(series(egf, x, n+1), x, n):
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MATHEMATICA
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nn = 20; a = Log[1/(1 - x)]/2 - x/2 - x^2/4; Drop[Range[0, nn]! CoefficientList[Series[x D[Exp[a], x], {x, 0, nn}], x], 3]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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