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| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 2 |
| 2 | 48 |
| 3 | 540 |
| 4 | 4480 |
| 5 | 31500 |
| 6 | 199584 |
| 7 | 1177176 |
| 8 | 6589440 |
| 9 | 35443980 |
| 10 | 184756000 |
| 11 | 938929992 |
| 12 | 4672781568 |
| 13 | 22850118200 |
| 14 | 110079950400 |
| 15 | 523521630000 |
| 16 | 2462025277440 |
| 17 | 11465007358860 |
| 18 | 52926189069600 |
| 19 | 242433164404200 |
| 20 | 1102772230560000 |
| 21 | 4984806175188840 |
| 22 | 22404445765690560 |
[0,2,48,540,4480,31500,199584,1177176,6589440, 35443980,184756000,938929992,4672781568, 22850118200,110079950400,523521630000, 2462025277440,11465007358860,52926189069600, 242433164404200,1102772230560000,4984806175188840, 22404445765690560] |