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A109456
as a simple table
n
a(n)
0
0
1
1
2
1
3
2
4
3
5
7
6
21
7
135
8
2470
9
319124
10
1214554343
11
1706241214185942
[0,1,1,2,3,7,21,135,2470,319124,1214554343, 1706241214185942]