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A181620
as a simple table
n
a(n)
1
2
2
4
3
31
4
91
5
183
6
4411
7
29611
8
59935
9
110791
10
10418851
11
658653031
12
20123369491
[2,4,31,91,183,4411,29611,59935,110791,10418851, 658653031,20123369491]