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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 100 |
| 5 | 650 |
| 6 | 4908 |
| 7 | 41090 |
| 8 | 382520 |
| 9 | 3863808 |
| 10 | 42409440 |
| 11 | 497972112 |
| 12 | 6259762320 |
| 13 | 83343114504 |
| 14 | 1175904241848 |
| 15 | 17442325040520 |
| 16 | 272149555445760 |
| 17 | 4438451554802880 |
| 18 | 75714874759039104 |
| 19 | 1343817666163911168 |
| 20 | 24837691533530152320 |
| 21 | 475811860099666527360 |
[1,1,4,18,100,650,4908,41090,382520,3863808, 42409440,497972112,6259762320,83343114504, 1175904241848,17442325040520,272149555445760, 4438451554802880,75714874759039104, 1343817666163911168,24837691533530152320, 475811860099666527360] |