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A156445
as a simple table
n
a(n)
2
1
3
4
4
18
5
138
6
3910
7
446508
8
89959563
9
25250999564
10
10095521906313
11
5572579847718200
12
4311704697906480578
[1,4,18,138,3910,446508,89959563,25250999564, 10095521906313,5572579847718200, 4311704697906480578]