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| n | a(n) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 6 |
| 4 | 28 |
| 5 | 170 |
| 6 | 988 |
| 7 | 7896 |
| 8 | 60492 |
| 9 | 555264 |
| 10 | 5819904 |
| 11 | 61776000 |
| 12 | 725950080 |
| 13 | 9894493440 |
| 14 | 137963243520 |
| 15 | 1875645434880 |
| 16 | 33258387456000 |
| 17 | 528975488563200 |
| 18 | 9760969019289600 |
| 19 | 175565885864140800 |
| 20 | 3608256006957772800 |
| 21 | 72367669059194880000 |
| 22 | 1745463407406243840000 |
[0,1,6,28,170,988,7896,60492,555264,5819904, 61776000,725950080,9894493440,137963243520, 1875645434880,33258387456000,528975488563200, 9760969019289600,175565885864140800, 3608256006957772800,72367669059194880000, 1745463407406243840000] |