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A214821
as a simple table
n
a(n)
1
1
2
3
3
6
4
20
5
57
6
240
7
954
8
4566
9
22641
10
121823
11
683307
12
4004055
[1,3,6,20,57,240,954,4566,22641,121823,683307, 4004055]