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 A036771 Number of labeled rooted trees with a degree constraint: ((3*n)!/(6^n)) * binomial(3*n + 1, n). 3
 1, 4, 420, 201600, 264264000, 734557824000, 3723191087616000, 31125877492469760000, 399532678960326912000000, 7462849882264211635200000000, 194563959280510261541299200000000, 6847568575944052279580806348800000000, 316573366618757452963440048714547200000000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS a(n) is the number of labeled rooted unordered trees on 3n+1 nodes such that each node has zero or three children. - Geoffrey Critzer, Mar 14 2013 LINKS INRIA Algorithms Project, Encyclopedia of Combinatorial Structures 47. Lajos Takacs, Enumeration of rooted trees and forests, Math. Scientist 18 (1993), 1-10; see Eqs. (12) and (13) on p. 4. Wikipedia, Cubic function. FORMULA E.g.f. (with interpolated zeros): -(1/2)/x * ((-3*x + ((-8 + 9*x^3) / x)^(1/2)) * x^2)^(1/3) - 1/((-3*x + ((-8 + 9*x^3) / x)^(1/2)) * x^2)^(1/3) - (1/2)*I*3^(1/2) * (1/x * ((-3*x + ((-8 + 9*x^3) / x)^(1/2)) * x^2)^(1/3) - 2/((-3*x + ((-8 + 9*x^3) / x)^(1/2)) * x^2)^(1/3)). Recurrence (with interpolated zeros): Define sequence (b(n): n >= 0) by b(3*n + 1) = a(n) for n >= 0 and b(n) = 0 otherwise. Then it satisfies the recurrence (-9*n^4 - 45*n^3 - 63*n^2 - 27*n) * b(n) + (8*n + 28) * b(n+3) = 0 for n >= 0 with b(0) = 0, b(1) = 1, and b(2) = 0. [Corrected by Petros Hadjicostas, Jun 07 2019] E.g.f. with interpolated zeros satisfies: A(x) = x*(1 + A(x)^3/3!). -  Geoffrey Critzer, Mar 14 2013 From Petros Hadjicostas, Jun 07 2019: (Start) In other words, if A(x) = Sum_{n >= 0} b(n)*x^n/n! = Sum_{m >= 0} a(m)*x^(3*m+1)/(3*m+1)!, then  A(x) = x*(1 + A(x)^3/3!). E.g.f. of (b(n): n >= 1) according to the link for ECS 47 above: Let f(x) = ((-3*x + sqrt((9*x^3 - 8)/x)) * x^2)^(1/3). Then the e.g.f. of (b(n): n >= 1), which includes interpolated zeros, is -f(x)/(2*x) - 1/f(x) - (I*sqrt(3)/2)*(f(x)/x - 2/f(x)). (It is not clear whether it is correct or useful.) E.g.f. using the solution of a cubic in terms of trigonometric functions: A(x) = (2*sqrt(2)/sqrt(|x|)) * cos( (1/3) * arccos((-3*|x|/2) * sqrt(|x|/2)) - 2*Pi/3 ) for 0 < |x| < 2/9^(1/3). (We have lim_{x -> 0} A(x) = 0.) (End) Recurrence without interpolated zeros: -3 * (3*n + 4) * (3*n + 1) * (3*n + 2)^2 * a(n) + 4 * (2*n + 3) * a(n + 1) = 0 for n >= 0 with a(0) = 1. - Petros Hadjicostas, Jun 08 2019 EXAMPLE E.g.f. (with interpolated zeros): 1*x/1! + 4*x^4/4! + 420*x^7/7! + 201600*x^10/10! + 264264000*x^13/13! + 734557824000*x^16/16! + 3723191087616000*x^19/19! + ... = x + 1/6*x^4 + 1/12*x^7 + 1/18*x^10 + 55/1296*x^13 + 91/2592*x^16 + 119/3888*x^19 + ... - Petros Hadjicostas, Jun 07 2019 MAPLE spec := [S, {S=Union(Z, Prod(Z, Set(S, card=3)))}, labeled]: seq(combstruct[count](spec, size=n), n=0..20); A := proc(x) 2*sqrt(2)*cos(1/3*arccos(-3/2*x*sqrt(1/2*x)) - 2/3*Pi)/sqrt(x); end proc; series(A(x), x = 0, 80);  # Petros Hadjicostas, Jun 07 2019 MATHEMATICA nn=34; f[x_]:=Sum[a[n]x^n/n!, {n, 0, nn}]; s=SolveAlways[0=Series[f[x]-x (1+f[x]^3/3!), {x, 0, nn}], x]; Table[a[n], {n, 1, nn, 3}]/.s  (* Geoffrey Critzer, Mar 14 2013 *) CROSSREFS Cf. A036770, A036772, A036773. Sequence in context: A287965 A215647 A228242 * A201980 A080321 A264469 Adjacent sequences:  A036768 A036769 A036770 * A036772 A036773 A036774 KEYWORD nonn AUTHOR STATUS approved

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Last modified June 17 00:17 EDT 2021. Contains 345080 sequences. (Running on oeis4.)