A018103 - OEIS

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A018103 as a simple table

n	a(n)
0	1
1	2
2	4
3	9
4	20
5	42
6	89
7	189
8	400
9	846
10	1789
11	3783
12	8000
13	16918
14	35777
15	75659
16	160000
17	338359
18	715542
19	1513187
20	3200000
21	6767176
22	14310835
23	30263731
24	64000000
25	135343522
26	286216701
27	605274630
28	1280000000

[1,2,4,9,20,42,89,189,400,846,1789,3783,8000, 16918,35777,75659,160000,338359,715542,1513187, 3200000,6767176,14310835,30263731,64000000, 135343522,286216701,605274630,1280000000]