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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 2 |
| 3 | 8 |
| 4 | 42 |
| 5 | 258 |
| 6 | 1824 |
| 7 | 14664 |
| 8 | 132360 |
| 9 | 1326120 |
| 10 | 14606640 |
| 11 | 175448160 |
| 12 | 2282469840 |
| 13 | 31972303440 |
| 14 | 479793807360 |
| 15 | 7679384173440 |
| 16 | 130586660507520 |
| 17 | 2351111258805120 |
| 18 | 44679858911251200 |
| 19 | 893744703503769600 |
| 20 | 18771276190401504000 |
| 21 | 413017883356110278400 |
[1,1,2,8,42,258,1824,14664,132360,1326120, 14606640,175448160,2282469840,31972303440, 479793807360,7679384173440,130586660507520, 2351111258805120,44679858911251200, 893744703503769600,18771276190401504000, 413017883356110278400] |