A384256 a(n) = Product_{k=0..n-1} (n+3*k+1).
1, 2, 18, 280, 6160, 174960, 6086080, 250490240, 11904278400, 641472832000, 38645634227200, 2573895458534400, 187787322731008000, 14894027431162880000, 1275931456704672768000, 117412145664335441920000, 11550258696757088788480000, 1209613643310990696210432000
Offset: 0
Programs
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PARI
a(n) = prod(k=0, n-1, n+3*k+1);
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Sage
def a(n): return 3^n*rising_factorial((n+1)/3, n)
Formula
a(n) = 3^n * RisingFactorial((n+1)/3,n).
a(n) = n! * [x^n] 1/(1 - 3*x)^((n+1)/3).