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 A211421 Integral factorial ratio sequence: a(n) = (8*n)!*(3*n)!/((6*n)!*(4*n)!*n!). 10
 1, 14, 390, 12236, 404550, 13777764, 478273692, 16825310040, 597752648262, 21397472070260, 770557136489140, 27884297395587240, 1013127645555452700, 36935287875280348776, 1350441573221798941560, 49498889739033621986736, 1818284097150186829038150 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS This sequence is the particular case a = 4, b = 3 of the following result (see Bober, Theorem 1.2): let a, b be nonnegative integers with a > b and GCD(a,b) = 1. Then (2*a*n)!*(b*n)!/((a*n)!*(2*b*n)!*((a-b)*n)!) is an integer for all integer n >= 0. Other cases include A061162 (a = 3, b = 1), A211419 (a = 3, b = 2) and A211420 (a = 4, b = 1). Sequence terms are given by the coefficient of x^n in the expansion of ( (1 + x)^(k+2)/(1 - x)^k )^n when k = 6. See the cross references for related sequences obtained from other values of k. - Peter Bala, Sep 29 2015 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..600 J. W. Bober, Factorial ratios, hypergeometric series, and a family of step functions, 2007, arXiv:0709.1977v1 [math.NT], 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 The o.g.f. sum {n >= 1} a(n)*z^n is algebraic over the field of rational functions Q(z) (see Rodriguez-Villegas). From Peter Bala, Sep 29 2015: (Start) a(n) = Sum_{i = 0..n} binomial(8*n,i)*binomial(7*n-i-1,n-i). a(n) = [x^n] ( (1 + x)^8/(1 - x)^6 )^n. a(0) = 1 and a(n) = 2*(8*n - 1)*(8*n - 3)*(8*n - 5)*(8*n - 7)/( n*(6*n - 1)*(6*n - 3)*(6*n - 5) ) * a(n-1) for n >= 1. exp( Sum_{n >= 1} a(n)*x^n/n ) = 1 + 14*x + 293*x^2 + 7266*x^3 + 197962*x^4 + 5726364*x^5 + ... has integer coefficients and equals 1/x * series reversion of x*(1 - x)^6/(1 + x)^8. See A262740. (End) a(n) ~ 2^(10*n)*27^(-n)/sqrt(2*Pi*n). - Ilya Gutkovskiy, Jul 31 2016 MAPLE a := n -> (8*n)!*(3*n)!/((6*n)!*(4*n)!*n!); seq(a(n), n = 0..16); MATHEMATICA Table[(8 n)!*(3 n)!/((6 n)!*(4 n)!*n!), {n, 0, 15}] (* Michael De Vlieger, Oct 04 2015 *) PROG (PARI) a(n) = (8*n)!*(3*n)!/((6*n)!*(4*n)!*n!); vector(30, n, a(n-1)) \\ Altug Alkan, Oct 02 2015 (MAGMA) [Factorial(8*n)*Factorial(3*n)/(Factorial(6*n)*Factorial(4*n)*Factorial(n)): n in [0..20]]; // Vincenzo Librandi, Aug 01 2016 CROSSREFS Cf. A061162, A061163, A211419, A211420. Cf. A000984 (k = 0), A091527 (k = 1), A001448 (k = 2), A262732 (k = 3), A211419 (k = 4), A262733 (k = 5), A262740. Sequence in context: A270409 A000473 A233094 * A097310 A223003 A228185 Adjacent sequences:  A211418 A211419 A211420 * A211422 A211423 A211424 KEYWORD nonn,easy AUTHOR Peter Bala, Apr 10 2012 STATUS approved

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Last modified January 25 23:08 EST 2020. Contains 331270 sequences. (Running on oeis4.)