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 A136281 Number of graphs on n labeled nodes with degree at most 2. 8
 1, 2, 8, 41, 253, 1858, 15796, 152219, 1638323, 19467494, 252998224, 3568259503, 54263159347, 884834059454, 15397757661092, 284767413357977, 5576696746139689, 115269732256964626, 2507575465491619672, 57262481225957071721, 1369461739453440893261 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS These are thunderstorm graphs.  Their connected components are a single cycle (clouds), a path (lightning bolts) or an isolated vertex (raindrops). - Geoffrey Critzer, May 11 2011 REFERENCES D. E. Knuth, The Art of Computer Programming, Vol. 4A, Section 7.1.4. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..200 Samuele Giraudo, Combalgebraic structures on decorated cliques, Formal Power Series and Algebraic Combinatorics, SÃ©minaire Lotharingien de Combinatoire, 78B.15, 2017, p. 8, arXiv:1709.08416 [math.CO], 2017. FORMULA Binomial transform of A000986. E.g.f.: (1-x)^(-1/2)*exp(-x^2/4 + x/((2*(1-x)))). - Vladeta Jovovic, May 20 2008 a(n) = (2*n-1)*a(n-1) - (n-1)^2*a(n-2) + (n-2)*(n-1)*a(n-3) - (n-3)*(n-2)*(n-1)/2*a(n-4). - Vaclav Kotesovec, Aug 10 2013 a(n) ~ n^n*exp(sqrt(2*n)-1/2-n)/sqrt(2) * (1+19/(24*sqrt(2*n))). - Vaclav Kotesovec, Aug 10 2013 MATHEMATICA f = (Log[1/(1-x)]+1/(1-x) -x^2/2 - 1)/2; Range[0, 25]! CoefficientList[Series[Exp[f], {x, 0, 25}], x] (* Geoffrey Critzer, May 11 2011 *) CROSSREFS Cf. A000085 (degree at most 1), A136282-A136286. Sequence in context: A093935 A099240 A134055 * A125698 A231495 A052447 Adjacent sequences:  A136278 A136279 A136280 * A136282 A136283 A136284 KEYWORD nonn AUTHOR Don Knuth, Mar 31 2008 EXTENSIONS More terms from Vladeta Jovovic, May 20 2008 STATUS approved

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Last modified November 28 07:44 EST 2020. Contains 338702 sequences. (Running on oeis4.)