A155456 - OEIS

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A155456 as a simple table

n	a(n)
0	-1
1	-1
2	1
3	-2
4	6
5	-24
6	120
7	-720
8	5040
9	-40320
10	362880
11	-3628800
12	39916800
13	-479001600
14	6227020800
15	-87178291200
16	1307674368000
17	-20922789888000
18	355687428096000
19	-6402373705728000
20	121645100408832000

[-1,-1,1,-2,6,-24,120,-720,5040,-40320,362880, -3628800,39916800,-479001600,6227020800, -87178291200,1307674368000,-20922789888000, 355687428096000,-6402373705728000, 121645100408832000]