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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 24 |
| 5 | 108 |
| 6 | 516 |
| 7 | 2556 |
| 8 | 12972 |
| 9 | 66996 |
| 10 | 350628 |
| 11 | 1854252 |
| 12 | 9888924 |
| 13 | 53107236 |
| 14 | 286882740 |
| 15 | 1557510012 |
| 16 | 8492587596 |
| 17 | 46483203348 |
| 18 | 255273601860 |
| 19 | 1406078670924 |
| 20 | 7765563869436 |
| 21 | 42991470093060 |
| 22 | 238528474655316 |
| 23 | 1326059132006556 |
[0,1,2,6,24,108,516,2556,12972,66996,350628, 1854252,9888924,53107236,286882740,1557510012, 8492587596,46483203348,255273601860,1406078670924, 7765563869436,42991470093060,238528474655316, 1326059132006556] |