A012997 - OEIS

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A012997 as a simple table

n	a(n)
0	1
1	2
2	4
3	8
4	16
5	72
6	544
7	3040
8	11008
9	64896
10	1060224
11	10595840
12	38065152
13	87854080
14	9708670976
15	170663275520
16	276991311872
17	-13108265091072
18	283899182678016
19	9548887177330688
20	-9456062364418048

[1,2,4,8,16,72,544,3040,11008,64896,1060224, 10595840,38065152,87854080,9708670976, 170663275520,276991311872,-13108265091072, 283899182678016,9548887177330688, -9456062364418048]