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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 28 |
| 4 | 264 |
| 5 | 3206 |
| 6 | 47684 |
| 7 | 839249 |
| 8 | 17058688 |
| 9 | 393216567 |
| 10 | 10134918592 |
| 11 | 288815780665 |
| 12 | 9016571143680 |
| 13 | 306027510946208 |
| 14 | 11219450971161024 |
| 15 | 441846991480590475 |
| 16 | 18602901833071633792 |
| 17 | 833832341625621777368 |
| 18 | 39642569136740054367808 |
[1,1,4,28,264,3206,47684,839249,17058688, 393216567,10134918592,288815780665,9016571143680, 306027510946208,11219450971161024, 441846991480590475,18602901833071633792, 833832341625621777368,39642569136740054367808] |