A020037 - OEIS

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A020037 as a simple table

n	a(n)
0	1
1	1
2	1
3	4
4	15
5	74
6	427
7	2901
8	22624
9	199095
10	1951127
11	21072175
12	248651668
13	3182741351
14	43921830648
15	650043093588
16	10270680878694
17	172547438762056
18	3071344409964594
19	57741274907334370
20	1143277243165220534

[1,1,1,4,15,74,427,2901,22624,199095,1951127, 21072175,248651668,3182741351,43921830648, 650043093588,10270680878694,172547438762056, 3071344409964594,57741274907334370, 1143277243165220534]