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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 14 |
| 4 | 66 |
| 5 | 384 |
| 6 | 2640 |
| 7 | 20880 |
| 8 | 186480 |
| 9 | 1854720 |
| 10 | 20321280 |
| 11 | 243129600 |
| 12 | 3153427200 |
| 13 | 44068147200 |
| 14 | 660064204800 |
| 15 | 10548573235200 |
| 16 | 179151388416000 |
| 17 | 3222109642752000 |
| 18 | 61178237632512000 |
| 19 | 1222853377794048000 |
| 20 | 25667116186263552000 |
| 21 | 564433265896980480000 |
| 22 | 12977099311614197760000 |
[1,1,4,14,66,384,2640,20880,186480,1854720, 20321280,243129600,3153427200,44068147200, 660064204800,10548573235200,179151388416000, 3222109642752000,61178237632512000, 1222853377794048000,25667116186263552000, 564433265896980480000,12977099311614197760000] |