A064649 - OEIS

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A064649 as a simple table

n	a(n)
1	1
2	4
3	12
4	40
5	140
6	816
7	5082
8	40800
9	363258
10	3632880
11	39916910
12	479052528
13	6227020956
14	87178936992
15	1307674429440
16	20922800222848
17	355687428096272
18	6402373892575992
19	121645100408832342
20	2432902011892837920

[1,4,12,40,140,816,5082,40800,363258,3632880, 39916910,479052528,6227020956,87178936992, 1307674429440,20922800222848,355687428096272, 6402373892575992,121645100408832342, 2432902011892837920]