A004661 - OEIS

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A004661 as a simple table

n	a(n)
0	1
1	3
2	12
3	36
4	144
5	465
6	2061
7	6243
8	25062
9	111246
10	334104
11	1335315
12	4342251
13	16360113
14	55440342
15	232651356
16	1031614434
17	3125446635
18	12413006541
19	40542026253
20	152256115122
21	520134351366
22	2160436414464

[1,3,12,36,144,465,2061,6243,25062,111246,334104, 1335315,4342251,16360113,55440342,232651356, 1031614434,3125446635,12413006541,40542026253, 152256115122,520134351366,2160436414464]