A012948 - OEIS

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A012948 as a simple table

n	a(n)
0	1
1	2
2	4
3	11
4	40
5	177
6	934
7	5665
8	39064
9	301209
10	2572314
11	24156365
12	247234332
13	2751134557
14	33003190406
15	426920246945
16	5905698391952
17	87530592395793
18	1378247307547506
19	23127717388170197
20	409930747848171812

[1,2,4,11,40,177,934,5665,39064,301209,2572314, 24156365,247234332,2751134557,33003190406, 426920246945,5905698391952,87530592395793, 1378247307547506,23127717388170197, 409930747848171812]