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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 96 |
| 5 | 624 |
| 6 | 4896 |
| 7 | 45072 |
| 8 | 474624 |
| 9 | 5619456 |
| 10 | 73903104 |
| 11 | 1069106688 |
| 12 | 16872800256 |
| 13 | 288483876864 |
| 14 | 5311773904896 |
| 15 | 104789944829952 |
| 16 | 2205099306123264 |
| 17 | 49302137885884416 |
| 18 | 1167150882577711104 |
| 19 | 29165495002777387008 |
| 20 | 767163772371852066816 |
| 21 | 21188300138891474632704 |
[1,4,18,96,624,4896,45072,474624,5619456,73903104, 1069106688,16872800256,288483876864,5311773904896, 104789944829952,2205099306123264, 49302137885884416,1167150882577711104, 29165495002777387008,767163772371852066816, 21188300138891474632704] |