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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 28 |
| 4 | 124 |
| 5 | 848 |
| 6 | 5312 |
| 7 | 40080 |
| 8 | 367632 |
| 9 | 3132096 |
| 10 | 27731328 |
| 11 | 474979008 |
| 12 | 1130161728 |
| 13 | 90279554688 |
| 14 | 268809015168 |
| 15 | 3005011325952 |
| 16 | 473192066191104 |
| 17 | -7913323872693504 |
| 18 | 186235895195313408 |
| 19 | 1357401816746159616 |
| 20 | -181477915903332002304 |
| 21 | 9552839425392612096000 |
[1,2,6,28,124,848,5312,40080,367632,3132096, 27731328,474979008,1130161728,90279554688, 268809015168,3005011325952,473192066191104, -7913323872693504,186235895195313408, 1357401816746159616,-181477915903332002304, 9552839425392612096000] |