| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 2 |
| 2 | 6 |
| 3 | 18 |
| 4 | 60 |
| 5 | 200 |
| 6 | 760 |
| 7 | 2888 |
| 8 | 11856 |
| 9 | 48672 |
| 10 | 215904 |
| 11 | 957728 |
| 12 | 4506304 |
| 13 | 21203072 |
| 14 | 105494400 |
| 15 | 524880000 |
| 16 | 2737670400 |
| 17 | 14279148032 |
| 18 | 77836363264 |
| 19 | 424289980928 |
| 20 | 2405307227136 |
| 21 | 13635728197632 |
| 22 | 80188215392256 |
| 23 | 471566299547648 |
| 24 | 2867649768509440 |
| 25 | 17438513317683200 |
[1,2,6,18,60,200,760,2888,11856,48672,215904, 957728,4506304,21203072,105494400,524880000, 2737670400,14279148032,77836363264,424289980928, 2405307227136,13635728197632,80188215392256, 471566299547648,2867649768509440, 17438513317683200] |