A007028 - OEIS

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A007028 as a simple table

n	a(n)
8	1
9	0
10	1
11	1
12	5
13	5
14	21
15	40
16	176
17	500
18	2053
19	7532
20	31206
21	124552
22	521332
23	2167599
24	9183879
25	38965327
26	166956753
27	718004565
28	3107450143
29	13502835176
30	58956369552
31	258398671525

[1,0,1,1,5,5,21,40,176,500,2053,7532,31206,124552, 521332,2167599,9183879,38965327,166956753, 718004565,3107450143,13502835176,58956369552, 258398671525]