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A307180
as a simple table
n
a(n)
0
1
1
0
2
1
3
2
4
6
5
15
6
68
7
310
8
2096
9
16554
10
152497
11
1574652
12
17947667
13
223013311
[1,0,1,2,6,15,68,310,2096,16554,152497,1574652, 17947667,223013311]