OFFSET
1,2
LINKS
R. H. Hardin, Table of n, a(n) for n = 1..33
FORMULA
a(n) = 720^(-n)*n!(3n)! Sum_{i=0..n} Sum_{j=0..n-i} Sum_{k=0..n-i-j} (15^(n-i-k)*45^k*(6i+4j+2k)!/(i!j!k!(n-i-j-k)!(2j+k+3i)!*2^(2j+k+3i))). - Shanzhen Gao, Feb 24 2010
a(n) ~ sqrt(Pi) * 3^(4*n + 1/2) * n^(6*n + 1/2) / (2^(n-1) * 5^n * exp(6*n - 5/2)). - Vaclav Kotesovec, Oct 22 2023
MATHEMATICA
Table[720^(-n)*n!*(3*n)! * Sum[Sum[Sum[(15^(n-i-k)*45^k*(6*i+4*j+2*k)! / (i!*j!*k!*(n-i-j-k)!*(2*j+k+3*i)! * 2^(2*j+k+3*i))), {k, 0, n-i-j}], {j, 0, n-i}], {i, 0, n}], {n, 1, 15}] (* Vaclav Kotesovec, Oct 22 2023 *)
PROG
(PARI) a(n) = 720^(-n)*n!*(3*n)!*sum(i=0, n, sum(j=0, n-i, sum(k=0, n-i-j, (15^(n-i-k)*45^k*(6*i+4*j+2*k)!/(i!*j!*k!*(n-i-j-k)!*(2*j+k+3*i)!*2^(2*j+k+3*i)))))) \\ Michel Marcus, Jan 17 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
R. H. Hardin, Feb 06 2010
STATUS
approved