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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 30 |
| 4 | 348 |
| 5 | 5460 |
| 6 | 108000 |
| 7 | 2578800 |
| 8 | 72189600 |
| 9 | 2319166080 |
| 10 | 84119968800 |
| 11 | 3400710112800 |
| 12 | 151635740256000 |
| 13 | 7393393739571840 |
| 14 | 391334037586652160 |
| 15 | 22347717728195059200 |
| 16 | 1369596926501682278400 |
| 17 | 89664110210432082124800 |
| 18 | 6245228229359073860505600 |
[1,1,4,30,348,5460,108000,2578800,72189600, 2319166080,84119968800,3400710112800, 151635740256000,7393393739571840, 391334037586652160,22347717728195059200, 1369596926501682278400,89664110210432082124800, 6245228229359073860505600] |