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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 21 |
| 4 | 140 |
| 5 | 1130 |
| 6 | 10674 |
| 7 | 115206 |
| 8 | 1396016 |
| 9 | 18739080 |
| 10 | 275712840 |
| 11 | 4408612560 |
| 12 | 76070179272 |
| 13 | 1408041937848 |
| 14 | 27816773482848 |
| 15 | 583970117197320 |
| 16 | 12978149959718400 |
| 17 | 304310928180279360 |
| 18 | 7506092106055537344 |
[1,1,4,21,140,1130,10674,115206,1396016,18739080, 275712840,4408612560,76070179272,1408041937848, 27816773482848,583970117197320,12978149959718400, 304310928180279360,7506092106055537344] |