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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 1 |
| 3 | 9 |
| 4 | 42 |
| 5 | 139 |
| 6 | 381 |
| 7 | 984 |
| 8 | 2685 |
| 9 | 8061 |
| 10 | 25434 |
| 11 | 79695 |
| 12 | 242577 |
| 13 | 721584 |
| 14 | 2131785 |
| 15 | 6333633 |
| 16 | 18984618 |
| 17 | 57194883 |
| 18 | 172319157 |
| 19 | 517851144 |
| 20 | 1552599333 |
| 21 | 4651054101 |
| 22 | 13939132698 |
| 23 | 41810229351 |
| 24 | 125475990057 |
| 25 | 376585031520 |
| 26 | 1129975049169 |
| 27 | 3389800055481 |
| 28 | 10168040440746 |
[0,0,1,9,42,139,381,984,2685,8061,25434,79695, 242577,721584,2131785,6333633,18984618,57194883, 172319157,517851144,1552599333,4651054101, 13939132698,41810229351,125475990057,376585031520, 1129975049169,3389800055481,10168040440746] |