A384168 a(n) = 3^n * n! * binomial(4*n/3,n) * Sum_{k=1..n} 1/(n+3*k).
1, 13, 234, 5566, 165944, 5966136, 251491120, 12169996912, 665146831680, 40530954643840, 2724842629685120, 200361647815660800, 15997170878205905920, 1378271357428552115200, 127459020533529062246400, 12593128815600367187507200, 1323895109721239722075136000
Offset: 1
Keywords
Programs
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PARI
a(n) = sum(k=0, n, k*(n+3)^(k-1)*3^(n-k)*abs(stirling(n, k, 1)));
Formula
a(n) = Sum_{k=0..n} k * (n+3)^(k-1) * 3^(n-k) * |Stirling1(n,k)|.
a(n) = n! * [x^n] ( -log(1 - 3*x)/(3 * (1 - 3*x)^(n/3+1)) ).