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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 24 |
| 4 | 140 |
| 5 | 992 |
| 6 | 8456 |
| 7 | 84224 |
| 8 | 957840 |
| 9 | 12257792 |
| 10 | 174293856 |
| 11 | 2726068224 |
| 12 | 46514037440 |
| 13 | 859790139392 |
| 14 | 17115342333056 |
| 15 | 365041325441024 |
| 16 | 8304761365213440 |
| 17 | 200743379109281792 |
| 18 | 5137820023434733056 |
| 19 | 138802611894431514624 |
| 20 | 3947233200665413667840 |
[1,2,6,24,140,992,8456,84224,957840,12257792, 174293856,2726068224,46514037440,859790139392, 17115342333056,365041325441024,8304761365213440, 200743379109281792,5137820023434733056, 138802611894431514624,3947233200665413667840] |