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A004987 a(n) = (3^n/n!)*Product_{k=0..n-1} (3*k + 1). 18
1, 3, 18, 126, 945, 7371, 58968, 480168, 3961386, 33011550, 277297020, 2344420260, 19927572210, 170150808870, 1458435504600, 12542545339560, 108179453553705, 935434098376155, 8107095519260010, 70403724246205350 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Diagonal of rational function R(x,y) = (1 - 9*x*y) / (1 - 2*x - 3*y + 3*y^2 + 9*x^2*y). - Gheorghe Coserea, Jul 01 2016

LINKS

Gheorghe Coserea, Table of n, a(n) for n = 0..200

A. Bostan, S. Boukraa, J.-M. Maillard, J.-A. Weil, Diagonals of rational functions and selected differential Galois groups, arXiv preprint arXiv:1507.03227 [math-ph], 2015.

A. Straub, V. H. Moll, T. Amdeberhan, The p-adic valuation of k-central binomial coefficients, Acta Arith. 140 (2009) 31-41, eq (1.10).

FORMULA

G.f.: (1 - 9*x)^(-1/3).

a(n) = (3^n/n!)*A007559(n), n >= 1, a(0) := 1.

a(n) ~ Gamma(1/3)^-1*n^(-2/3)*3^(2*n)*{1 - 1/9*n^-1 + ...}.

Representation as n-th moment of a positive function on (0, 9): a(n)=int(x^n*(1/(Pi*sqrt(3)*6*(x/9)^(2/3)*(1-x/9)^(1/3))), x=0..9), n=0, 1,.... This function is the solution of the Hausdorff moment problem on (0, 9) with moments equal to a(n). As a consequence this representation is unique. - Karol A. Penson, Jan 30 2003

n*a(n) + 3*(2-3*n)*a(n-1)=0. - R. J. Mathar, Jun 07 2013

0 = a(n) * (81*a(n+1) - 15*a(n+2)) + a(n+1) * (-3*a(n+1) + a(n+2)) for all n in Z. - Michael Somos, Jan 27 2014

G.f. A(x)=:y satisfies 0 = y'' * y - 4 * y' * y'. - Michael Somos, Jan 27 2014

a(n) = (-9)^n*binomial(-1/3,n). - Peter Luschny, Mar 23 2014

E.g.f.: is the hypergeometric function of type 1F1, in Maple notation hypergeom([1/3],[1],9*x). - Karol A. Penson, Dec 19 2015

Sum_{n>=0} 1/a(n) = (sqrt(3)*Pi + 3*(12 + log(3)))/32 = 1.3980385924595932... - Ilya Gutkovskiy, Jul 01 2016

EXAMPLE

G.f.: 1 + 3*x + 18*x^2 + 126*x^3 + 945*x^4 + 7371*x^5 + 58968*x^6 + 480168*x^7 + ...

MATHEMATICA

Table[(-9)^n Binomial[-1/3, n], {n, 0, 19}] (* Jean-Fran├žois Alcover, Sep 28 2016, after Peter Luschny *)

PROG

(PARI) a(n) = prod(k=0, n-1, 3*k + 1)*3^n/n! \\ Michel Marcus, Jun 30 2013

(PARI)

my(x='x, y='y);

R = (1 - 9*x*y) / (1 - 2*x - 3*y + 3*y^2 + 9*x^2*y);

diag(n, expr, var) = {

  my(a = vector(n));

  for (i = 1, #var, expr = taylor(expr, var[#var - i + 1], n));

  for (k = 1, n, a[k] = expr;

       for (i = 1, #var, a[k] = polcoeff(a[k], k-1)));

  return(a);

};

diag(20, R, [x, y])  \\ Gheorghe Coserea, Jul 01 2016

CROSSREFS

Cf. A004117, A007559, A047657, A054861, A034689, A053101, A072888.

Related to diagonal of rational functions: A268545-A268555.

Sequence in context: A181998 A176277 A289429 * A074557 A073971 A264230

Adjacent sequences:  A004984 A004985 A004986 * A004988 A004989 A004990

KEYWORD

nonn

AUTHOR

Joe Keane (jgk(AT)jgk.org)

EXTENSIONS

More terms from Ralf Stephan, Mar 13 2004

More terms from Benoit Cloitre, Jun 05 2004

STATUS

approved

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Last modified February 20 12:33 EST 2018. Contains 299379 sequences. (Running on oeis4.)