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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 28 |
| 3 | 220 |
| 4 | 1820 |
| 5 | 15504 |
| 6 | 134596 |
| 7 | 1184040 |
| 8 | 10518300 |
| 9 | 94143280 |
| 10 | 847660528 |
| 11 | 7669339132 |
| 12 | 69668534468 |
| 13 | 635013559600 |
| 14 | 5804731963800 |
| 15 | 53194089192720 |
| 16 | 488526937079580 |
| 17 | 4495151581425648 |
| 18 | 41432089765583440 |
| 19 | 382460951663844400 |
[1,4,28,220,1820,15504,134596,1184040,10518300, 94143280,847660528,7669339132,69668534468, 635013559600,5804731963800,53194089192720, 488526937079580,4495151581425648, 41432089765583440,382460951663844400] |