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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 120 |
| 5 | 780 |
| 6 | 5382 |
| 7 | 38638 |
| 8 | 285762 |
| 9 | 2162033 |
| 10 | 16655167 |
| 11 | 130193037 |
| 12 | 1030117023 |
| 13 | 8234025705 |
| 14 | 66391916397 |
| 15 | 539360587341 |
| 16 | 4410492096741 |
| 17 | 36274113675369 |
| 18 | 299864297741292 |
| 19 | 2490192142719336 |
| 20 | 20764402240048267 |
| 21 | 173784940354460219 |
| 22 | 1459360304511145146 |
[0,1,4,20,120,780,5382,38638,285762,2162033, 16655167,130193037,1030117023,8234025705, 66391916397,539360587341,4410492096741, 36274113675369,299864297741292,2490192142719336, 20764402240048267,173784940354460219, 1459360304511145146] |