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A061540
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Number of connected labeled graphs with n nodes and n+1 edges.
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5
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0, 0, 0, 6, 205, 5700, 156555, 4483360, 136368414, 4432075200, 154060613850, 5720327205120, 226378594906035, 9523895202838016, 424814409531910125, 20037831121798963200, 996964614369038858060, 52198565072252054814720
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OFFSET
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1,4
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REFERENCES
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Miklos Bona, editor, Handbook of Enumerative Combinatorics, CRC Press, 2015, p. 407, Eq. (6.5).
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LINKS
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FORMULA
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E.g.f.: W1(x) := T(x)^4/24 * (6-T(x))/(1-T(x))^3 where T(x) is the e.g.f. for rooted labeled trees (A000169), i.e. T(x) = -LambertW(-x) = x*exp(T(x)).
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MAPLE
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add(binomial(n, s)*s^s*(n-s)^(n-s), s=1..n-1) ;
end proc:
(n-1)*(5*n^2+3*n+2)*n^(n-2)-14*A001864(n) ;
%/24 ;
end proc: # R. J. Mathar, May 10 2016 see Chapter 6.3 in Bona's Handbook of Combinatorics
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MATHEMATICA
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max = 18; t[x_] := -ProductLog[-x]; w1[x_] := t[x]^4/24*(6-t[x])/(1-t[x])^3; Drop[ CoefficientList[ Series[ w1[x], {x, 0, max}], x]*Range[0, max]!, 1] (* Jean-François Alcover, Apr 02 2012, after e.g.f. *)
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PROG
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(Python)
from math import comb
def A061540(n): return 0 if n<2 else ((n*(n*(5*n - 2) - 1) - 2)*n**(n-2)-14*((sum(comb(n, k)*(n-k)**(n-k)*k**k for k in range(1, (n+1>>1)))<<1) + (0 if n&1 else comb(n, m:=n>>1)*m**n)))//24 # Chai Wah Wu, Apr 26 2023
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CROSSREFS
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KEYWORD
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easy,nice,nonn
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AUTHOR
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RAVELOMANANA Vlady (vlad(AT)lri.fr), May 16 2001
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STATUS
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approved
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