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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 264 |
| 4 | 2860 |
| 5 | 33592 |
| 6 | 416024 |
| 7 | 5348880 |
| 8 | 70715340 |
| 9 | 955277400 |
| 10 | 13128240840 |
| 11 | 182965127280 |
| 12 | 2579808294648 |
| 13 | 36734706144304 |
| 14 | 527495903500720 |
| 15 | 7629973004184608 |
| 16 | 111068129754096396 |
| 17 | 1625888084299461528 |
| 18 | 23919596771720906984 |
| 19 | 353467725574013402800 |
[1,4,28,264,2860,33592,416024,5348880,70715340, 955277400,13128240840,182965127280,2579808294648, 36734706144304,527495903500720,7629973004184608, 111068129754096396,1625888084299461528, 23919596771720906984,353467725574013402800] |