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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 30 |
| 4 | 240 |
| 5 | 2125 |
| 6 | 19776 |
| 7 | 192129 |
| 8 | 1922496 |
| 9 | 19692504 |
| 10 | 205444500 |
| 11 | 2175519379 |
| 12 | 23322637440 |
| 13 | 252631900235 |
| 14 | 2760767859780 |
| 15 | 30400169155500 |
| 16 | 336977763170048 |
| 17 | 3757141504436392 |
| 18 | 42107201575798248 |
| 19 | 474084628585822412 |
| 20 | 5359833703935374000 |
| 21 | 60823006052351537106 |
| 22 | 692556314455384443196 |
[1,4,30,240,2125,19776,192129,1922496,19692504, 205444500,2175519379,23322637440,252631900235, 2760767859780,30400169155500,336977763170048, 3757141504436392,42107201575798248, 474084628585822412,5359833703935374000, 60823006052351537106,692556314455384443196] |