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A323875
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Number of labeled graphs on n nodes with two connected components.
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3
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0, 1, 3, 19, 230, 5098, 207536, 15891372, 2343580752, 675458276144, 383306076989440, 430041136692146912, 956431386434331323776, 4224539434553753578497024, 37106501188130085159785113344, 648740172906485727983524271405824, 22591360806791558877526051411343415296, 1567817808096346724727108606144936617617408
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OFFSET
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1,3
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REFERENCES
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F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, 1973, section 1.2.
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LINKS
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Table of n, a(n) for n=1..18.
Marko Riedel et al., Proof of recurrence relation.
Marko Riedel, Maple implementation of memoized recurrence.
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FORMULA
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a(n) = A143543(n, 2).
E.g.f.: log(Sum_{q>=0} 2^binomial(q, 2)*z^q/q!)^2/2!.
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CROSSREFS
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Cf. A143543, A323876, A323877.
Sequence in context: A198046 A295812 A228229 * A001929 A349962 A230316
Adjacent sequences: A323872 A323873 A323874 * A323876 A323877 A323878
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KEYWORD
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nonn
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AUTHOR
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Marko Riedel, Feb 05 2019
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STATUS
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approved
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