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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 2 |
| 2 | 27 |
| 3 | 224 |
| 4 | 1500 |
| 5 | 8910 |
| 6 | 49049 |
| 7 | 256256 |
| 8 | 1288872 |
| 9 | 6298500 |
| 10 | 30093910 |
| 11 | 141210432 |
| 12 | 652860520 |
| 13 | 2981331990 |
| 14 | 13472983125 |
| 15 | 60343756800 |
| 16 | 268187306640 |
| 17 | 1183875281820 |
| 18 | 5194996380090 |
| 19 | 22676052360000 |
| 20 | 98513956031400 |
| 21 | 426171522716940 |
| 22 | 1836562780483002 |
[0,2,27,224,1500,8910,49049,256256,1288872, 6298500,30093910,141210432,652860520,2981331990, 13472983125,60343756800,268187306640, 1183875281820,5194996380090,22676052360000, 98513956031400,426171522716940,1836562780483002] |