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| n | a(n) |
|---|---|
| 1 | 4 |
| 2 | 20 |
| 3 | 78 |
| 4 | 420 |
| 5 | 2088 |
| 6 | 9916 |
| 7 | 49740 |
| 8 | 245904 |
| 9 | 1204720 |
| 10 | 5966528 |
| 11 | 29461128 |
| 12 | 145208512 |
| 13 | 717277504 |
| 14 | 3540859200 |
| 15 | 17473224048 |
| 16 | 86264173904 |
| 17 | 425824598480 |
| 18 | 2101839903376 |
| 19 | 10375467545712 |
| 20 | 51215782794640 |
| 21 | 252809675084016 |
| 22 | 1247933698121232 |
[4,20,78,420,2088,9916,49740,245904,1204720, 5966528,29461128,145208512,717277504,3540859200, 17473224048,86264173904,425824598480, 2101839903376,10375467545712,51215782794640, 252809675084016,1247933698121232] |