A138272 - OEIS

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A138272 as a simple table

n	a(n)
0	1
1	1
2	4
3	28
4	268
5	3164
6	43672
7	682632
8	11834536
9	224283416
10	4597330748
11	101100605540
12	2370095338372
13	58924635472260
14	1546999678599872
15	42734731811302304
16	1238309697485975648
17	37538075865014728096

[1,1,4,28,268,3164,43672,682632,11834536, 224283416,4597330748,101100605540,2370095338372, 58924635472260,1546999678599872,42734731811302304, 1238309697485975648,37538075865014728096]