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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 0 |
| 3 | 1 |
| 4 | 2 |
| 5 | 6 |
| 6 | 18 |
| 7 | 54 |
| 8 | 168 |
| 9 | 543 |
| 10 | 1819 |
| 11 | 6284 |
| 12 | 22305 |
| 13 | 81085 |
| 14 | 301185 |
| 15 | 1140759 |
| 16 | 4398268 |
| 17 | 17237268 |
| 18 | 68582448 |
| 19 | 276723505 |
| 20 | 1131245762 |
| 21 | 4681484414 |
| 22 | 19597666672 |
| 23 | 82933899715 |
| 24 | 354575503930 |
| 25 | 1530739734801 |
| 26 | 6669583434025 |
| 27 | 29316248173094 |
[1,0,1,2,6,18,54,168,543,1819,6284,22305,81085, 301185,1140759,4398268,17237268,68582448, 276723505,1131245762,4681484414,19597666672, 82933899715,354575503930,1530739734801, 6669583434025,29316248173094] |