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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 0 |
| 2 | 1 |
| 3 | 4 |
| 4 | 15 |
| 5 | 56 |
| 6 | 208 |
| 7 | 768 |
| 8 | 2823 |
| 9 | 10352 |
| 10 | 37944 |
| 11 | 139232 |
| 12 | 512048 |
| 13 | 1888896 |
| 14 | 6992960 |
| 15 | 25989888 |
| 16 | 96983687 |
| 17 | 363368672 |
| 18 | 1366820944 |
| 19 | 5160846912 |
| 20 | 19556183352 |
| 21 | 74352602304 |
| 22 | 283560228000 |
| 23 | 1084470001024 |
[0,0,1,4,15,56,208,768,2823,10352,37944,139232, 512048,1888896,6992960,25989888,96983687, 363368672,1366820944,5160846912,19556183352, 74352602304,283560228000,1084470001024] |