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| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 1 |
| 3 | 6 |
| 4 | 27 |
| 5 | 168 |
| 6 | 1395 |
| 7 | 12744 |
| 8 | 136521 |
| 9 | 1672704 |
| 10 | 22659993 |
| 11 | 340472160 |
| 12 | 5605239123 |
| 13 | 100154133504 |
| 14 | 1933748430939 |
| 15 | 40097756280960 |
| 16 | 888588043228305 |
| 17 | 20962928129900544 |
| 18 | 524479633529596209 |
| 19 | 13871161113800394240 |
[1,1,1,6,27,168,1395,12744,136521,1672704, 22659993,340472160,5605239123,100154133504, 1933748430939,40097756280960,888588043228305, 20962928129900544,524479633529596209, 13871161113800394240] |