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a(n) = (7*n)!*(n/2)!/((7*n/2)!*(n)!*(3*n)!).
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%I #19 Oct 05 2023 08:37:22

%S 1,64,12012,2621440,608435100,146028888064,35794148650260,

%T 8901646138474496,2237242000722428700,566823049100850626560,

%U 144520856111821003326512,37036782455383679028953088,9531607276865293630675462980,2461693334077582876433071472640

%N a(n) = (7*n)!*(n/2)!/((7*n/2)!*(n)!*(3*n)!).

%C Row 4 of A364519.

%H Paolo Xausa, <a href="/A364520/b364520.txt">Table of n, a(n) for n = 0..400</a>

%F a(n) = [x^(3*n)] ( (1 + x)^7/(1 - x) )^n.

%F a(n) = Sum_{j = 0..3*n} binomial(7*n, j)*binomial(4*n-j-1, 3*n-j).

%F a(n) = binomial(4*n-1, 3*n) * hypergeom([-7*n, -3*n], [1 - 4*n], -1) for n >= 2.

%F a(n) ~ c^n * 1/sqrt(6*Pi*n) where c = (14/3)^3*sqrt(7).

%F P-recursive: a(n) = 448*(7*n-1)*(7*n-3)*(7*n-5)*(7*n-9)*(7*n-11)*(7*n-13)/(3*n*(3*n-1)*(3*n-2)*(3*n-3)*(3*n-4)*(3*n-5)) * a(n-2) with a(0) = 1 and a(1) = 64.

%F The generating function is an algebraic function over the field of rational functions Q(x).

%p seq( simplify((7*n)!*(n/2)!/((7*n/2)!*(n)!*(3*n)!)), n = 0..15);

%t A364520[n_]:=(7n)!(n/2)!/((7n/2)!n!(3n)!);Array[A364520,15,0] (* _Paolo Xausa_, Oct 05 2023 *)

%o (Python)

%o from math import factorial

%o from sympy import factorial2

%o def A364520(n): return int((factorial(7*n)*factorial2(n)<<(3*n))//(factorial2(7*n)*factorial(n)*factorial(3*n))) # _Chai Wah Wu_, Aug 13 2023

%Y Cf. A276098, A364519.

%K nonn,easy

%O 0,2

%A _Peter Bala_, Aug 09 2023