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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 28 |
| 5 | 180 |
| 6 | 1502 |
| 7 | 15456 |
| 8 | 189208 |
| 9 | 2683920 |
| 10 | 43263962 |
| 11 | 780807456 |
| 12 | 15593180788 |
| 13 | 341340941760 |
| 14 | 8126644655222 |
| 15 | 209050212857856 |
| 16 | 5777935570510768 |
| 17 | 170755837008595200 |
| 18 | 5373097909706399282 |
| 19 | 179351443518333574656 |
| 20 | 6329687401322560131148 |
| 21 | 235491796312126982538240 |
[1,2,6,28,180,1502,15456,189208,2683920,43263962, 780807456,15593180788,341340941760,8126644655222, 209050212857856,5777935570510768, 170755837008595200,5373097909706399282, 179351443518333574656,6329687401322560131148, 235491796312126982538240] |