A159319 a(n) = 3^(n^2+n) * C(2*n-1 + 1/3^n, n) / (n*3^n + 1).

1, 3, 126, 66708, 379033074, 21399656315607, 11566324342205917416, 58678275719834357303044728, 2762222169999029718435709903699050, 1197781369953334546750963984948238943438411

Offset

0,2

Links

G. C. Greubel, Table of n, a(n) for n = 0..45

Formula

G.f.: A(x) = Sum_{n>=0} a(n)*x^n/3^(n^2+n).

G.f.: A(x) = Sum_{n>=0} log(F(x/3^n))^n/n! and

a(n)/3^(n^2+n) = [x^n] F(x)^(1/3^n) where

F(x) = (1-sqrt(1-4*x))/(2*x) is the Catalan function (A000108).

a(n)/3^(n^2+n) = [x^n] 1/(1-x)^(n + 1/3^n)/(n*3^n + 1).

Radius of convergence of series A(x) is |x| <= 3/4.

Example

G.f.: A(x) = 1 + 3*x/3^2 + 126*x^2/3^6 + 66708*x^3/3^12 + 379033074*x^4/3^20 +...
A(x) = Sum_{n>=0} log( (1-sqrt(1-4*x/3^n))/(2*x/3^n) )^n/n!.
A(x) = 1 + log(F(x/3)) + log(F(x/9))^2/2! + log(F(x/27))^3/3! +... where F(x) = (1-sqrt(1-4*x))/(2*x).
Special values.
A(3/4) = 1 + log(2) + log(6-6*sqrt(2/3))^2/2! + log(18-18*sqrt(8/9))^3/3! + log(54-54*sqrt(26/27))^4/4! +...
A(3/4) = 1.6977820781412737038286578011417848301231627494589650...
A(-3/4) = 1 + log(2*sqrt(2)-2) + log(6*sqrt(4/3)-6)^2/2! + log(18*sqrt(10/9)-18)^3/3! + log(54*sqrt(28/27)-54)^4/4! +...
A(-3/4) = 0.8145458917316632938137444904602229430460096517471900...
Illustrate (3^n)-th root formula:
a(n)/3^(n^2+n) = [x^n] F(x)^(1/3^n) or, equivalently,
a(n) = [x^n] F(3^(n+1)*x)^(1/3^n) where F(x)=Catalan(x):
F(3*x) = (1) + 3*x + 18*x^2 + 135*x^3 + 1134*x^4 + 10206*x^5 +...
F(9*x)^(1/3) = 1 + (3)*x + 45*x^2 + 936*x^3 + 22572*x^4 +...
F(27*x)^(1/9) = 1 + 3*x + (126)*x^2 + 7659*x^3 + 546480*x^4 +...
F(81*x)^(1/27) = 1 + 3*x + 369*x^2 + (66708)*x^3 + 14215230*x^4 +...
F(243*x)^(1/81) = 1 + 3*x + 1098*x^2 + 593775*x^3 + (379033074)*x^4 +...
coefficients in parenthesis form the initial terms of this sequence.

Mathematica

Table[3^(n^2 +n)*Binomial[2*n -1 +1/3^n, n]/(n*3^n +1), {n, 0, 50}] (* G. C. Greubel, Jun 26 2018 *)

Prog

(PARI) {a(n)=3^(n^2+n)*binomial(2*n-1+1/3^n, n)/(n*3^n + 1)}
(PARI) {a(n)=3^(n^2+n)*polcoeff(1/(1-x+x*O(x^n))^(n+1/3^n)/(n*3^n + 1), n)}
(PARI) {a(n)=3^(n^2+n)*polcoeff(((1-sqrt(1-4*x+x^2*O(x^n)))/(2*x))^(1/3^n), n)}
(PARI) {a(n)=3^(n^2+n)*polcoeff(sum(k=0, n, log((1-sqrt(1-4*x/3^k+x^2*O(x^n)))/(2*x/3^k))^k/k!), n)}
(Magma) [3^(n^2 +n)*Binomial(2*n -1 +1/3^n, n)/(n*3^n +1): n in [0..40]]; // G. C. Greubel, Jun 26 2018

Crossrefs

Cf. A159318, A158093, A159558, A159478, A000108.

Sequence in context: A274314 A157592 A213988 * A086154 A133122 A139936

Adjacent sequences: A159316 A159317 A159318 * A159320 A159321 A159322

Keyword

nonn

Author

Paul D. Hanna, Apr 23 2009

Status

approved