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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 20 |
| 4 | 84 |
| 5 | 360 |
| 6 | 1848 |
| 7 | 9360 |
| 8 | 55440 |
| 9 | 318240 |
| 10 | 2106720 |
| 11 | 13366080 |
| 12 | 96909120 |
| 13 | 668304000 |
| 14 | 5233092480 |
| 15 | 38761632000 |
| 16 | 324451733760 |
| 17 | 2558267712000 |
| 18 | 22711621363200 |
| 19 | 189311810688000 |
| 20 | 1771506466329600 |
| 21 | 15523568476416000 |
| 22 | 152349556104345600 |
| 23 | 1397121162877440000 |
[1,2,6,20,84,360,1848,9360,55440,318240,2106720, 13366080,96909120,668304000,5233092480, 38761632000,324451733760,2558267712000, 22711621363200,189311810688000,1771506466329600, 15523568476416000,152349556104345600, 1397121162877440000] |