A123476 - OEIS

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A123476 as a simple table

n	a(n)
1	1
2	4
3	18
4	72
5	450
6	2700
7	22050
8	176400
9	1587600
10	15876000
11	192099600
12	2305195200
13	32464832400
14	454507653600
15	6817614804000
16	109081836864000
17	1970290678356000
18	35465232210408000
19	711274934886516000

[1,4,18,72,450,2700,22050,176400,1587600,15876000, 192099600,2305195200,32464832400,454507653600, 6817614804000,109081836864000,1970290678356000, 35465232210408000,711274934886516000]