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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 16 |
| 3 | 64 |
| 4 | 280 |
| 5 | 1504 |
| 6 | 9856 |
| 7 | 70144 |
| 8 | 498136 |
| 9 | 3449440 |
| 10 | 23506816 |
| 11 | 160566784 |
| 12 | 1115048896 |
| 13 | 7905796864 |
| 14 | 56994288640 |
| 15 | 414928113664 |
| 16 | 3034880623576 |
| 17 | 22255957312864 |
| 18 | 163667338903936 |
| 19 | 1208070406612480 |
| 20 | 8955840250934080 |
| 21 | 66678657938510080 |
[1,4,16,64,280,1504,9856,70144,498136,3449440, 23506816,160566784,1115048896,7905796864, 56994288640,414928113664,3034880623576, 22255957312864,163667338903936,1208070406612480, 8955840250934080,66678657938510080] |