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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 2 |
| 3 | 4 |
| 4 | 31 |
| 5 | 172 |
| 6 | 1154 |
| 7 | 8044 |
| 8 | 58595 |
| 9 | 439514 |
| 10 | 3379972 |
| 11 | 26513202 |
| 12 | 211413199 |
| 13 | 1709183316 |
| 14 | 13981482114 |
| 15 | 115537553904 |
| 16 | 963234521237 |
| 17 | 8093123460146 |
| 18 | 68468543003422 |
| 19 | 582819983344618 |
| 20 | 4988538052577712 |
| 21 | 42911585401063684 |
[0,2,4,31,172,1154,8044,58595,439514,3379972, 26513202,211413199,1709183316,13981482114, 115537553904,963234521237,8093123460146, 68468543003422,582819983344618,4988538052577712, 42911585401063684] |