A017959 - OEIS

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A017959 as a simple table

n	a(n)
0	1
1	4
2	18
3	76
4	324
5	1375
6	5832
7	24743
8	104976
9	445375
10	1889568
11	8016758
12	34012224
13	144301645
14	612220032
15	2597429617
16	11019960576
17	46753733110
18	198359290368
19	841567195983
20	3570467226624
21	15148209527701

[1,4,18,76,324,1375,5832,24743,104976,445375, 1889568,8016758,34012224,144301645,612220032, 2597429617,11019960576,46753733110,198359290368, 841567195983,3570467226624,15148209527701]