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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 0 |
| 2 | 1 |
| 3 | 4 |
| 4 | 21 |
| 5 | 115 |
| 6 | 712 |
| 7 | 5027 |
| 8 | 40299 |
| 9 | 362846 |
| 10 | 3628745 |
| 11 | 39916711 |
| 12 | 479001456 |
| 13 | 6227020567 |
| 14 | 87178290823 |
| 15 | 1307674367390 |
| 16 | 20922789887013 |
| 17 | 355687428094403 |
| 18 | 6402373705725416 |
| 19 | 121645100408827819 |
| 20 | 2432902008176633235 |
| 21 | 51090942171709429054 |
[1,0,1,4,21,115,712,5027,40299,362846,3628745, 39916711,479001456,6227020567,87178290823, 1307674367390,20922789887013,355687428094403, 6402373705725416,121645100408827819, 2432902008176633235,51090942171709429054] |