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A118581
as a simple table
n
a(n)
0
1
1
2
2
7
3
31
4
219
5
2134
6
30768
7
1658440
8
3685688857
9
105981863625149
[1,2,7,31,219,2134,30768,1658440,3685688857, 105981863625149]