A121639 - OEIS

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A121639 as a simple table

n	a(n)
1	0
2	1
3	5
4	25
5	147
6	996
7	7668
8	66264
9	635976
10	6717600
11	77482080
12	969338880
13	13076778240
14	189261999360
15	2925629280000
16	48111515827200
17	838731380659200
18	15451544605593600
19	299960798422118400
20	6120505381423104000

[0,1,5,25,147,996,7668,66264,635976,6717600, 77482080,969338880,13076778240,189261999360, 2925629280000,48111515827200,838731380659200, 15451544605593600,299960798422118400, 6120505381423104000]