A384257 a(n) = Product_{k=0..n-1} (n+3*k+2).
1, 3, 28, 440, 9720, 276640, 9634240, 396809280, 18866848000, 1016990374400, 61283225203200, 4082333102848000, 297880548623257600, 23628360309345792000, 2024347339040266240000, 186294495108985303040000, 18327479444105919639552000, 1919453757320555804508160000
Offset: 0
Programs
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PARI
a(n) = prod(k=0, n-1, n+3*k+2);
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Sage
def a(n): return 3^n*rising_factorial((n+2)/3, n)
Formula
a(n) = 3^n * RisingFactorial((n+2)/3,n).
a(n) = n! * [x^n] 1/(1 - 3*x)^((n+2)/3).