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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 0 |
| 3 | 1 |
| 4 | 4 |
| 5 | 20 |
| 6 | 108 |
| 7 | 694 |
| 8 | 5142 |
| 9 | 43193 |
| 10 | 406016 |
| 11 | 4222569 |
| 12 | 48137291 |
| 13 | 596902408 |
| 14 | 7998492273 |
| 15 | 115178288736 |
| 16 | 1773745646539 |
| 17 | 29089428603255 |
| 18 | 506156057696641 |
| 19 | 9313271461618195 |
| 20 | 180677466355392995 |
| 21 | 3685820313650017111 |
[1,0,0,1,4,20,108,694,5142,43193,406016,4222569, 48137291,596902408,7998492273,115178288736, 1773745646539,29089428603255,506156057696641, 9313271461618195,180677466355392995, 3685820313650017111] |