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A091496 a(n) = ((5n)!/(n!(2n)!))(gamma(1+n/2)/gamma(1+5n/2)). 4
1, 16, 630, 28672, 1385670, 69206016, 3528923580, 182536110080, 9540949030470, 502682972323840, 26651569523959380, 1420217179365703680, 75998432812419471900, 4081125953526124511232, 219813190240007470094520 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

Let a > b be nonnegative integers. Then the ratio of factorials ((2*a + 1)*n)!*((b + 1/2)*n)!/(((a + 1/2)*n)!*((2*b + 1)*n)!*((a - b)*n)!) is an integer for all integer n >= 0. This is the case a = 2, b = 0. - Peter Bala, Aug 28 2016

REFERENCES

R. P. Stanley, Enumerative Combinatorics Volume 2, Cambridge Univ. Press, 1999, Theorem 6.33, p. 197.

LINKS

G. C. Greubel, Table of n, a(n) for n = 0..500

P. Bala, Some integer ratios of factorials

FORMULA

n*(n-1)*(2*n-1)*(2*n-3)*a(n) = 20*(5*n-1)*(5*n-3)*(5*n-7)*(5*n-9)*a(n-2).

From Peter Bala, Aug 22 2016: (Start)

a(n) = Sum_{k = 0..2*n} (binomial(5*n,k)*binomial(3*n - k - 1,2*n - k).

a(n) = Sum_{k = 0..n} binomial(6*n, 2*n - 2*k)*binomial(n + k - 1, k).

a(n) ~ 5^(5*n/2)/(2*sqrt(Pi*n)).

O.g.f. A(x) = Hypergeom([9/10, 7/10, 3/10, 1/10], [3/4, 1/2, 1/4], 3125*x^2) + 16*x*Hypergeom([7/5, 6/5, 4/5, 3/5], [5/4, 3/2, 3/4], 3125*x^2).

a(n) = [x^(2*n)] H(x)^n, where H(x) = (1 + x)^5/(1 - x). Cf. A061162 and A262732.

It follows that the o.g.f. for this sequence is the diagonal of the bivariate rational generating function 1/2*( 1/(1 - t*H(sqrt(x))) + 1/(1 - t*H(-sqrt(x))) ) and hence is algebraic by Stanley 1999, Theorem 6.33, p. 197.

exp(Sum_{n >= 1} a(n)*x^n/n) = 1 + 16*x + 443*x^2 + 15280*x^3 + 591998*x^4 + 24635360*x^5 + 1075884051*x^6 + ... has integer coefficients.

Let F(x) = 1/x*Series_Reversion( x*sqrt((1 - x)/(1 + x)^5) ) and put G(x) = 1 + x*d/dx(Log(F(x)). Then A(x) satisfies A(x^2) = (G(x) + G(-x))/2. (End)

MATHEMATICA

Table[((5 n)!/(n! (2 n)!)) (Gamma[1 + n/2]/Gamma[1 + 5 n/2]), {n, 0, 14}] (* or *)

Table[Sum[Binomial[6 n, 2 n - 2 k] Binomial[n + k - 1, k], {k, 0, n}], {n, 0, 14}] (* or *)

Table[Sum[Binomial[5 n, k] Binomial[3 n - k - 1, 2 n - k], {k, 0, 2 n}], {n, 0, 14}] (* Michael De Vlieger, Aug 28 2016 *)

PROG

(PARI) a(n)=16^n*sum(i=0, 2*n, binomial(i-1+(n-1)/2, i))

CROSSREFS

Cf. A061163(n)=a(2n), A061162, A091527, A262732, A262733, A276098, A276099.

Sequence in context: A016792 A077204 A159745 * A139166 A128048 A197670

Adjacent sequences:  A091493 A091494 A091495 * A091497 A091498 A091499

KEYWORD

nonn

AUTHOR

Michael Somos, Jan 15 2004

STATUS

approved

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Last modified June 16 11:12 EDT 2019. Contains 324152 sequences. (Running on oeis4.)