A123385 a(n) = (n!)^2/2.

2, 18, 288, 7200, 259200, 12700800, 812851200, 65840947200, 6584094720000, 796675461120000, 114721266401280000, 19387894021816320000, 3800027228275998720000, 855006126362099712000000

Offset: 2

Author

Gary W. Adamson, Oct 13 2006

Keywords

• nonn

Links

• G. C. Greubel, Table of n, a(n) for n = 2..250

Programs

Magma

[(Factorial(n))^2/2: n in [2..25]]; // G. C. Greubel, Oct 13 2017

Mathematica

f[0] = 0; f[1] = 1; f[n_] := f[n] = Sum[Product[k, {k, i, n}]*f[i], {i, 0, n - 1}] Table[f[n], {n, 0, 30}]
Table[(n!)^2/2, {n, 2, 50}] (* G. C. Greubel, Oct 13 2017 *)

PARI

for(n=2,25, print1((n!)^2/2, ", ")) \\ G. C. Greubel, Oct 13 2017

Formula

a(n) = Sum[Product[k, {k, i, n}]*a(i), {i, 0, n - 1}].