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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 6 |
| 2 | 72 |
| 3 | 582 |
| 4 | 4032 |
| 5 | 25542 |
| 6 | 153048 |
| 7 | 881118 |
| 8 | 4925616 |
| 9 | 26909934 |
| 10 | 144356280 |
| 11 | 762839334 |
| 12 | 3981064368 |
| 13 | 20556000822 |
| 14 | 105173637672 |
| 15 | 533839505646 |
| 16 | 2690761186608 |
| 17 | 13478479905486 |
| 18 | 67142893855752 |
| 19 | 332807521103670 |
[0,6,72,582,4032,25542,153048,881118,4925616, 26909934,144356280,762839334,3981064368, 20556000822,105173637672,533839505646, 2690761186608,13478479905486,67142893855752, 332807521103670] |