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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 1 |
| 5 | 4 |
| 6 | 16 |
| 7 | 50 |
| 8 | 158 |
| 9 | 472 |
| 10 | 1406 |
| 11 | 4111 |
| 12 | 11998 |
| 13 | 34781 |
| 14 | 100660 |
| 15 | 290464 |
| 16 | 837137 |
| 17 | 2408914 |
| 18 | 6925100 |
| 19 | 19888057 |
| 20 | 57071610 |
[0,0,0,1,4,16,50,158,472,1406,4111,11998,34781, 100660,290464,837137,2408914,6925100,19888057, 57071610] |