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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 27 |
| 4 | 168 |
| 5 | 1140 |
| 6 | 7800 |
| 7 | 54845 |
| 8 | 390320 |
| 9 | 2815344 |
| 10 | 20494320 |
| 11 | 150442908 |
| 12 | 1111782672 |
| 13 | 8264558016 |
| 14 | 61743361680 |
| 15 | 463306724595 |
| 16 | 3489942222624 |
| 17 | 26378657835816 |
| 18 | 199991245341888 |
| 19 | 1520403553182800 |
| 20 | 11587257160313120 |
| 21 | 88506896001503616 |
| 22 | 677426230547667744 |
[0,1,4,27,168,1140,7800,54845,390320,2815344, 20494320,150442908,1111782672,8264558016, 61743361680,463306724595,3489942222624, 26378657835816,199991245341888,1520403553182800, 11587257160313120,88506896001503616, 677426230547667744] |