| n | a(n) |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 2 |
| 3 | 15 |
| 4 | 60 |
| 5 | 381 |
| 6 | 2286 |
| 7 | 16731 |
| 8 | 133848 |
| 9 | 1211193 |
| 10 | 12111930 |
| 11 | 133290279 |
| 12 | 1599483348 |
| 13 | 20793814965 |
| 14 | 291113409510 |
| 15 | 4366705925619 |
| 16 | 69867294809904 |
| 17 | 1187744054815089 |
| 18 | 21379392986671602 |
| 19 | 406208467134180927 |
| 20 | 8124169342683618540 |
[0,1,2,15,60,381,2286,16731,133848,1211193, 12111930,133290279,1599483348,20793814965, 291113409510,4366705925619,69867294809904, 1187744054815089,21379392986671602, 406208467134180927,8124169342683618540] |