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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 24 |
| 4 | 196 |
| 5 | 2040 |
| 6 | 25924 |
| 7 | 390264 |
| 8 | 6804676 |
| 9 | 135033720 |
| 10 | 3007364164 |
| 11 | 74315818104 |
| 12 | 2018441506756 |
| 13 | 59776933889400 |
| 14 | 1917312391176004 |
| 15 | 66216538949389944 |
| 16 | 2449977966210378436 |
| 17 | 96685769287005577080 |
| 18 | 4053944607498740773444 |
| 19 | 179973441341757042161784 |
| 20 | 8433644996370680262923716 |
[1,4,24,196,2040,25924,390264,6804676,135033720, 3007364164,74315818104,2018441506756, 59776933889400,1917312391176004,66216538949389944, 2449977966210378436,96685769287005577080, 4053944607498740773444,179973441341757042161784, 8433644996370680262923716] |