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A027321
as a simple table
n
a(n)
0
1
1
2
2
6
3
20
4
90
5
692
6
5774
7
48924
8
417440
9
3580036
10
30831756
11
266463804
[1,2,6,20,90,692,5774,48924,417440,3580036, 30831756,266463804]