A123854 - OEIS

A123854

Denominators in an asymptotic expansion for the cubic recurrence sequence A123851.

1, 4, 32, 128, 2048, 8192, 65536, 262144, 8388608, 33554432, 268435456, 1073741824, 17179869184, 68719476736, 549755813888, 2199023255552, 140737488355328, 562949953421312, 4503599627370496, 18014398509481984, 288230376151711744, 1152921504606846976

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OFFSET

0,2

COMMENTS

A cubic analog of the asymptotic expansion A116603 of Somos's quadratic recurrence sequence A052129. Numerators are A123853.

Equals 2^A004134(n); also the denominators in expansion of (1-x)^(-1/4). - Alexander Adamchuk, Oct 27 2006

All terms are powers of 2 and log_2 a(n) = A004134(n) = 3*n - A000120(n). - Alexander Adamchuk, Oct 27 2006 [Edited by Petros Hadjicostas, May 14 2020]

Is this the same sequence as A088802? - N. J. A. Sloane, Mar 21 2007

Almost certainly this is the same as A088802. - Michael Somos, Aug 23 2007

Denominators of Gegenbauer_C(2n,1/4,2). The denominators of Gegenbauer_C(n,1/4,2) give the doubled sequence. - Paul Barry, Apr 21 2009

If the Greubel formula in A088802 and the Luschny formula here are correct (they are the same), the sequence is a duplicate of A088802. - R. J. Mathar, Aug 02 2023

REFERENCES

Steven R. Finch, Mathematical Constants, Cambridge University Press, Cambridge, 2003, p. 446.

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..1000

T. M. Apostol, On the Lerch zeta function, Pacific J. Math. 1 (1951), 161-167. [In Eq. (3.7), p. 166, the index in the summation for the Apostol-Bernoulli numbers should start at s = 0, not at s = 1. - Petros Hadjicostas, Aug 09 2019]

Jonathan Sondow and Petros Hadjicostas, The generalized-Euler-constant function gamma(z) and a generalization of Somos's quadratic recurrence constant, arXiv:math/0610499 [math.CA], 2006.

Jonathan Sondow and Petros Hadjicostas, The generalized-Euler-constant function gamma(z) and a generalization of Somos's quadratic recurrence constant, J. Math. Anal. Appl. 332 (2007), 292-314.

Eric Weisstein's World of Mathematics, Somos's Quadratic Recurrence Constant.

Aimin Xu, Asymptotic expansion related to the Generalized Somos Recurrence constant, International Journal of Number Theory 15(10) (2019), 2043-2055. [The author gives recurrences and other formulas for the coefficients of the asymptotic expansion using the Apostol-Bernoulli numbers (see the reference above) and the Bell polynomials. - Petros Hadjicostas, Aug 09 2019]

FORMULA

From Alexander Adamchuk, Oct 27 2006: (Start)

a(n) = 2^A004134(n).

a(n) = 2^(3n - A000120(n)). (End)

a(n) = denominator(binomial(1/4,n)). - Peter Luschny, Apr 07 2016

EXAMPLE

A123851(n) ~ c^(3^n)*n^(- 1/2)/(1 + 3/(4*n) - 15/(32*n^2) + 113/(128*n^3) - 5397/(2048*n^4) + ...) where c = 1.1563626843322... is the cubic recurrence constant A123852.

MAPLE

f:=proc(t, x) exp(sum(ln(1+m*x)/t^m, m=1..infinity)); end; for j from 0 to 29 do denom(coeff(series(f(3, x), x=0, 30), x, j)); od;

# Alternatively:

A123854 := n -> denom(binomial(1/4, n)):

seq(A123854(n), n=0..25); # Peter Luschny, Apr 07 2016

MATHEMATICA

Denominator[CoefficientList[Series[ 1/Sqrt[Sqrt[1-x]], {x, 0, 25}], x]] (* Robert G. Wilson v, Mar 23 2014 *)

PROG

(Sage) # uses[A000120]

def A123854(n): return 1 << (3*n-A000120(n))

[A123854(n) for n in (0..25)] # Peter Luschny, Dec 02 2012

(PARI) vector(25, n, n--; denominator(binomial(1/4, n)) ) \\ G. C. Greubel, Aug 08 2019