A047139 - OEIS

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A047139 as a simple table

n	a(n)
0	1
1	2
2	9
3	48
4	261
5	1534
6	9319
7	56808
8	353362
9	2227380
10	14099553
11	89931668
12	577292621
13	3718701300
14	24047442913
15	156059054586
16	1015345651345
17	6621929417524
18	43283675452318
19	283439130488986
20	1859169844171611

[1,2,9,48,261,1534,9319,56808,353362,2227380, 14099553,89931668,577292621,3718701300, 24047442913,156059054586,1015345651345, 6621929417524,43283675452318,283439130488986, 1859169844171611]