A047891 - OEIS

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A047891 as a simple table

n	a(n)
1	1
2	3
3	12
4	57
5	300
6	1686
7	9912
8	60213
9	374988
10	2381322
11	15361896
12	100389306
13	663180024
14	4421490924
15	29712558576
16	201046204173
17	1368578002188
18	9366084668802
19	64403308499592
20	444739795023054
21	3082969991029800

[1,3,12,57,300,1686,9912,60213,374988,2381322, 15361896,100389306,663180024,4421490924, 29712558576,201046204173,1368578002188, 9366084668802,64403308499592,444739795023054, 3082969991029800]