A009686 - OEIS

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A009686 as a simple table

n	a(n)
0	0
1	1
2	2
3	6
4	32
5	232
6	1952
7	18928
8	211584
9	2677504
10	37670912
11	582734592
12	9835366400
13	179895500800
14	3544111063040
15	74812812851200
16	1684542923735040
17	40302006976086016
18	1020944014293204992

[0,1,2,6,32,232,1952,18928,211584,2677504, 37670912,582734592,9835366400,179895500800, 3544111063040,74812812851200,1684542923735040, 40302006976086016,1020944014293204992]