A006973 - OEIS

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A006973 as a simple table

n	a(n)
1	0
2	1
3	2
4	9
5	24
6	130
7	720
8	8505
9	35840
10	412776
11	3628800
12	42030450
13	479001600
14	7019298000
15	82614884352
16	1886805545625
17	20922789888000
18	374426276224000
19	6402373705728000
20	134987215801622184
21	2379913632645120000

[0,1,2,9,24,130,720,8505,35840,412776,3628800, 42030450,479001600,7019298000,82614884352, 1886805545625,20922789888000,374426276224000, 6402373705728000,134987215801622184, 2379913632645120000]