The OEIS is supported by the many generous donors to the OEIS Foundation.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 144 |
| 5 | 1245 |
| 6 | 12473 |
| 7 | 120213 |
| 8 | 1067757 |
| 9 | 8508432 |
| 10 | 60801152 |
| 11 | 391078610 |
| 12 | 2277606189 |
| 13 | 12079172109 |
| 14 | 58646808981 |
| 15 | 261900799332 |
| 16 | 1080219903328 |
| 17 | 4129996450120 |
| 18 | 14683917874810 |
| 19 | 48687786994190 |
| 20 | 150930310833366 |
[1,1,4,20,144,1245,12473,120213,1067757,8508432, 60801152,391078610,2277606189,12079172109, 58646808981,261900799332,1080219903328, 4129996450120,14683917874810,48687786994190, 150930310833366] |