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| n | a(n) |
|---|---|
| 1 | 1 |
| 2 | 4 |
| 3 | 28 |
| 4 | 180 |
| 5 | 2308 |
| 6 | 30028 |
| 7 | 502080 |
| 8 | 9458176 |
| 9 | 215401680 |
| 10 | 6387798300 |
| 11 | 200559384576 |
| 12 | 7369724839680 |
| 13 | 304250263527208 |
| 14 | 13082668722666720 |
| 15 | 611670442764457840 |
| 16 | 32588685419205242880 |
| 17 | 1922760350056947598944 |
| 18 | 117190066177425882515040 |
| 19 | 7810108077410021826572976 |
[1,4,28,180,2308,30028,502080,9458176,215401680, 6387798300,200559384576,7369724839680, 304250263527208,13082668722666720, 611670442764457840,32588685419205242880, 1922760350056947598944,117190066177425882515040, 7810108077410021826572976] |