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A347856 a(n) = (6*n)!/((4*n)!*n!) * (n/2)!/(3*n/2)!. 0
1, 20, 990, 56576, 3432198, 215147520, 13768454700, 893768826880, 58626071754822, 3876225891958784, 257898242928604740, 17245961314149335040, 1158088115444301759900, 78040346182201091555328 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Fractional factorials are defined using the Gamma function; for example, (n/2)! := Gamma(1 + n/2).

The sequence defined by u(n) = (12*n)!/((8*n)!*(2*n)!) * n!/(3*n)! is one of the 52 sporadic integral factorial ratio sequences of height 1 found by V. I. Vasyunin (see Bober, Table 2, Entry 3). See A295433. Here we are essentially considering the sequence (u(n/2))n>=0. The sequence is conjectured to be integral.

LINKS

Table of n, a(n) for n=0..13.

J. W. Bober, Factorial ratios, hypergeometric series, and a family of step functions, arXiv:0709.1977 [math.NT], 2007; J. London Math. Soc., Vol. 79, Issue 2 (2009), 422-444.

F. Rodriguez-Villegas, Integral ratios of factorials and algebraic hypergeometric functions, arXiv:math/0701362 [math.NT], 2007.

FORMULA

a(n) = binomial(6*n,4*n)*binomial(2*n,n)/binomial(3*n/2,n).

a(2*n) = A295433(n).

a(2*n) = 54*(6*n-1)*(6*n-5)*(12*n-1)*(12*n-5)*(12*n-7)*(12*n-11)/(n*(2*n-1)*(8*n-1)*(8*n-3)*(8*n-5)*(8*n-7))*a(2*n-2);

a(2*n+1) = 216*(9*n^2-1)*(144*n^2-1)*(144*n^2-5^2)/(n*(2*n+1)*(64*n^2-1)*(64*n^2-3^2))*a(2*n-1).

Asymptotics: a(n) ~ 1/(2*sqrt(n*Pi)) *3^(9*n/2)/2^n as n -> infinity.

O.g.f. A(x) = hypergeom([1/12, 1/6, 5/12, 7/12, 5/6, 11/12], [1/8, 3/8, 1/2, 5/8, 7/8], (19683/4)*x^2) + 20*x*hypergeom([17/12, 13/12, 11/12, 7/12, 4/3, 2/3], [11/8, 9/8, 7/8, 5/8, 3/2], (19683/4)*x^2) is conjectured to be algebraic over Q(x).

Conjectural congruences: a(n*p^k) == a(n*p^(k-1)) ( mod p^(3*k) ) for prime p >= 5 and positive integers n and k.

EXAMPLE

Congruence: a(11) - a(1) = 17245961314149335040 - 20 = (2^2)*5*(11^3)*103*6289876694707 == 0 (mod 11^3).

MAPLE

seq( (6*n)!/((4*n)!*n!) * GAMMA(1+n/2)/GAMMA(1+3*n/2), n = 0..12);

CROSSREFS

Cf. A295433, A347854 - A347858.

Sequence in context: A069578 A269644 A349964 * A128482 A128476 A160132

Adjacent sequences:  A347853 A347854 A347855 * A347857 A347858 A347870

KEYWORD

nonn,easy

AUTHOR

Peter Bala, Sep 17 2021

STATUS

approved

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Last modified January 17 17:47 EST 2022. Contains 350402 sequences. (Running on oeis4.)