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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 4 |
| 2 | 30 |
| 3 | 280 |
| 4 | 2925 |
| 5 | 32736 |
| 6 | 383838 |
| 7 | 4654320 |
| 8 | 57887550 |
| 9 | 734405100 |
| 10 | 9467075926 |
| 11 | 123648163392 |
| 12 | 1632743088275 |
| 13 | 21761329287600 |
| 14 | 292362576381900 |
| 15 | 3955219615609056 |
| 16 | 53834425161872586 |
| 17 | 736687428853685400 |
| 18 | 10129401435828605700 |
| 19 | 139876690363085200200 |
[1,4,30,280,2925,32736,383838,4654320,57887550, 734405100,9467075926,123648163392,1632743088275, 21761329287600,292362576381900,3955219615609056, 53834425161872586,736687428853685400, 10129401435828605700,139876690363085200200] |