A006403 - OEIS

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A006403 as a simple table

n	a(n)
1	0
2	1
3	2
4	3
5	6
6	15
7	36
8	114
9	396
10	1565
11	6756
12	31563
13	154370
14	785113
15	4099948
16	21870704
17	118624544
18	652485364
19	3631820462
20	20426666644
21	115949791342
22	663640383400

[0,1,2,3,6,15,36,114,396,1565,6756,31563,154370, 785113,4099948,21870704,118624544,652485364, 3631820462,20426666644,115949791342,663640383400]