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 A034940 Number of rooted labeled triangular cacti with 2n+1 nodes (n triangles). 7
 1, 3, 75, 5145, 688905, 152193195, 50174679555, 23089081640625, 14140034726843025, 11119632520038117075, 10920803043967635894075, 13100477280449146440878025, 18849023772776126861572265625, 32038907667175368299033846026875, 63516199119599233704934379969701875 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 REFERENCES F. Bergeron, G. Labelle and P. Leroux, Combinatorial Species and Tree-Like Structures, Camb. 1998, p. 307. (4.2.44) LINKS Alois P. Heinz, Table of n, a(n) for n = 0..100 Maryam Bahrani and Jérémie Lumbroso, Enumerations, Forbidden Subgraph Characterizations, and the Split-Decomposition, arXiv:1608.01465 [math.CO], 2016. Wikipedia, Lagrange Inversion Theorem FORMULA a(n) = b(2*n+1) where e.g.f. of b satisfies B(x)=x*exp(B(x)^2/2). The closed form a(n) = (2n-1)!! (2n+1)^n can be obtained from the generating function. - Noam D. Elkies, Dec 16 2002 From Peter Bala, Jul 31 2012: (Start) E.g.f. A(x) = series reversion of x*exp(-1/2*x^2) = sum {n >= 0} a(n)*x^(2*n+1)/(2*n+1)! = x + 3*x^3/3! + 75*x^5/5! + .... The Lagrange inversion formula gives a(n) = (2*n+1)^n*(2*n)!/(2^n*n!). A(x)^2 = T(x^2), where T denotes the tree function T(x) := sum {n >= 1} n^(n-1)*x^n/n!. A(x)^r = sum {n >= 0} r*(2*n+r)^(n-1)*x^(2*n+r)/(2^n*n!). x = A(x)*exp(-1/2*A(x)^2). dA/dx = exp(1/2*A^2)/(1-A^2). Let the function F(x) = A(exp(x)). Then dF/dx = F/(1-F^2). More generally, (d/dx)^(n+1)(F) is a rational function in F(x) given by (d/dx)^(n+1)(F) = F*R(n,F^2)/(1-F^2)^(2*n+1), where R(n,x) is the n-th row generating polynomial of A214406. (End) EXAMPLE E.g. a(3) = 5!! 7^3 = (1*3*5) * 343 = 5145. From Peter Bala, Jul 31 2012: (Start) Relation with rows of A214406: F(x) := A(exp(x)). (d/dx)^1(F) = F/(1-F^2) (d/dx)^2(F)) = F*(1 + F^2)/(1 - F^2)^3 (d/dx)^3(F)) = F*(1 + 8*F^2 + 3*F^4)/(1 - F^2)^5 (d/dx)^4(F)) = F*(1 + 33*F^2 + 71*F^4 + 15*F^6)/(1 - F^2)^7 (End) MATHEMATICA a[n_] := (2*n-1)!!*(2*n+1)^n; Table[a[n], {n, 0, 11}] (* Jean-François Alcover, May 13 2013, after Noam D. Elkies *) PROG (PARI) a(n) = (2*n+1)^n*(2*n)!/(2^n*n!); \\ Andrew Howroyd, Aug 30 2018 CROSSREFS Cf. A003080, A000169, A214406. Sequence in context: A228841 A249938 A195263 * A300924 A183290 A320306 Adjacent sequences:  A034937 A034938 A034939 * A034941 A034942 A034943 KEYWORD nonn,eigen AUTHOR Christian G. Bower, Oct 15 1998 EXTENSIONS a(10) corrected by Jean-François Alcover, May 13 2013 a(12)-a(14) from Alois P. Heinz, Jul 08 2015 STATUS approved

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Last modified March 18 16:00 EDT 2019. Contains 321292 sequences. (Running on oeis4.)