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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 15 |
| 3 | 135 |
| 4 | 531 |
| 5 | 8613 |
| 6 | 28161 |
| 7 | 477279 |
| 8 | 1539207 |
| 9 | 26178201 |
| 10 | 84335553 |
| 11 | 1434726783 |
| 12 | 4621740687 |
| 13 | 78627215025 |
| 14 | 253283346081 |
| 15 | 4308981442911 |
| 16 | 13880597644287 |
| 17 | 236143613924865 |
| 18 | 760693546670625 |
| 19 | 12941296146524511 |
| 20 | 41688023124390687 |
[0,15,135,531,8613,28161,477279,1539207,26178201, 84335553,1434726783,4621740687,78627215025, 253283346081,4308981442911,13880597644287, 236143613924865,760693546670625,12941296146524511, 41688023124390687] |