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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 24 |
| 3 | 120 |
| 4 | 620 |
| 5 | 3096 |
| 6 | 15600 |
| 7 | 77976 |
| 8 | 390480 |
| 9 | 1952380 |
| 10 | 9764880 |
| 11 | 48824280 |
| 12 | 244136904 |
| 13 | 1220683800 |
| 14 | 6103496400 |
| 15 | 30517481424 |
| 16 | 152587794020 |
| 17 | 762938966520 |
| 18 | 3814696782120 |
| 19 | 19073483892120 |
| 20 | 95367429207720 |
| 21 | 476837146020720 |
| 22 | 2384185778835696 |
| 23 | 11920928894086200 |
[1,4,24,120,620,3096,15600,77976,390480,1952380, 9764880,48824280,244136904,1220683800,6103496400, 30517481424,152587794020,762938966520, 3814696782120,19073483892120,95367429207720, 476837146020720,2384185778835696, 11920928894086200] |