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| n | a(n) |
|---|---|
| 0 | 3 |
| 1 | 15 |
| 2 | 93 |
| 3 | 660 |
| 4 | 3606 |
| 5 | 34560 |
| 6 | 236964 |
| 7 | 1559520 |
| 8 | 9155916 |
| 9 | 77423280 |
| 10 | 530807472 |
| 11 | 3868683120 |
| 12 | 21224771760 |
| 13 | 185094572580 |
| 14 | 1261494915594 |
| 15 | 9988783073280 |
| 16 | 49990612274316 |
| 17 | 436182213726030 |
| 18 | 3279858902194056 |
| 19 | 21372989348391720 |
| 20 | 122709716651985624 |
| 21 | 1082323574100172800 |
[3,15,93,660,3606,34560,236964,1559520,9155916, 77423280,530807472,3868683120,21224771760, 185094572580,1261494915594,9988783073280, 49990612274316,436182213726030,3279858902194056, 21372989348391720,122709716651985624, 1082323574100172800] |