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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 30 |
| 4 | 336 |
| 5 | 5100 |
| 6 | 97920 |
| 7 | 2275560 |
| 8 | 62092800 |
| 9 | 1946004480 |
| 10 | 68887324800 |
| 11 | 2718570254400 |
| 12 | 118349479987200 |
| 13 | 5634364354819200 |
| 14 | 291216292048281600 |
| 15 | 16240198896649728000 |
| 16 | 971984105567354880000 |
| 17 | 62145219629409857740800 |
| 18 | 4227385588071740430336000 |
[1,1,4,30,336,5100,97920,2275560,62092800, 1946004480,68887324800,2718570254400, 118349479987200,5634364354819200, 291216292048281600,16240198896649728000, 971984105567354880000,62145219629409857740800, 4227385588071740430336000] |